<PAGE>
                                                                     Exhibit 4.8


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                      PROTECTION ONE ALARM MONITORING, INC.
                                    as Issuer



                              PROTECTION ONE, INC.,
                       PROTECTION ONE INTERNATIONAL, INC.,
                      PROTECTION ONE INVESTMENTS, INC., and
                    NETWORK MULTI-FAMILY SECURITY CORPORATION

                                  as Guarantors



                                       and



                       STATE STREET BANK AND TRUST COMPANY

                                   as Trustee



                                   ----------



                          Supplemental Indenture No. 3

                          Dated as of February 14, 2000


                                   ----------

              6 3/4% Convertible Senior Subordinated Notes due 2003


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<PAGE>

     THIS SUPPLEMENTAL INDENTURE NO. 3, dated as of February 14, 2000, among
PROTECTION ONE ALARM MONITORING, INC., a Delaware corporation (the "Issuer"),
PROTECTION ONE, INC., a Delaware corporation (the "Parent Company"), PROTECTION
ONE INTERNATIONAL, INC., a Delaware corporation ("International"), PROTECTION
ONE INVESTMENTS, INC., a Delaware corporation ("Investments"), and NETWORK
MULTI-FAMILY SECURITY CORPORATION ("Network" together with the Parent Company,
International and Investments, the "Guarantors"), and STATE STREET BANK AND
TRUST COMPANY, a Massachusetts trust company, as trustee (the "Trustee") (the
"Supplemental Indenture").

                              W I T N E S S E T H:

     WHEREAS, the Issuer has issued 6 3/4% Convertible Senior Subordinated Notes
due 2003 (the "Notes") in the principal amount of $103,500,000, pursuant to that
certain Subordinated Debt Shelf Indenture dated as of August 29, 1996 providing
for the issuance from time to time of unsecured debentures, notes or other
evidences in one or more series of the Issuer (the "Subordinated Indenture") and
a Supplemental Indenture No. 1 dated as of September 20, 1996 (the "First
Supplement").

     WHEREAS, pursuant to Section 8.01 of the Subordinated Indenture, Security
Holdings, Inc., a Washington corporation ("Security Holdings"), the Issuer, the
Parent Company and the Trustee thereafter entered into a Supplemental Indenture
No. 2 dated as of October 28,1996, for the purpose of adding a Note Guarantee by
Security Holdings (the "Second Supplement"; the Subordinated Indenture, as
supplemented by the First Supplement and the Second Supplement, the
"Indenture");

     WHEREAS, in accordance with applicable provisions of the Indenture,
Security Holdings thereafter merged into Monitoring;

     WHEREAS, pursuant to Section 8.01 of the Subordinated Indenture, the
Issuer, the Guarantors and the Trustee may enter into a supplemental indenture
to (i) add additional Note Guarantees to the Indenture and (ii) reflect that
Security Holdings has ceased to be liable on its Note Guarantee because it is no
longer a subsidiary of Issuer;

     WHEREAS, the Issuer, the Guarantors and the Trustee desire to execute this
Supplemental Indenture to (i) provide for the Note Guarantees of International,
Investments and Network and (ii) reflect the merger of Security Holdings into
the Issuer;

     WHEREAS, Monitoring and the Guarantors have duly authorized the execution
and delivery of this Supplemental Indenture, the conditions set forth in the
Indenture for the execution and delivery of this Supplemental Indenture have
been satisfied, and all things necessary to make this Supplemental Indenture a
valid amendment of, and supplement to, the Indenture have been done by the
Issuer and the Guarantors.

<PAGE>

     NOW, THEREFORE, in consideration of the premises and the mutual covenants
herein, the Issuer and the Guarantors agree with the Trustee that the Indenture
is supplemented and amended, solely to the extent and for the purposes expressed
herein, for the equal and proportionate benefit of the Holders, as follows:

                                    ARTICLE I

                                   DEFINITIONS

     SECTION 1.1 Unless the context otherwise requires, the terms defined in the
Indenture shall, for all purposes of this Supplemental Indenture (including the
Recitals hereof) have the meanings therein defined.

                                   ARTICLE II

                           AMENDMENT OF THE INDENTURE

     SECTION 2.1 PREAMBLE. The preamble of the Subordinated Indenture is hereby
amended and restated in its entirety to read as follows:

          "THIS INDENTURE dated as of August 29, 1996 by and among PROTECTION
     ONE ALARM MONITORING, INC., a Delaware corporation (the "Issuer"), and
     PROTECTION ONE, INC., a Delaware corporation (the "Parent Company"),
     PROTECTION ONE INTERNATIONAL, INC., a Delaware corporation
     ("International"), PROTECTION ONE INVESTMENTS, INC., a Delaware corporation
     ("Investments"), NETWORK MULTI-FAMILY SECURITY CORPORATION, a Delaware
     corporation ("Network", and together with the Parent Company, International
     and Investments, the "Guarantors"), as Guarantors, and STATE STREET BANK
     AND TRUST COMPANY, a Massachusetts trust company, as trustee (the
     "Trustee")."

     SECTION 2.2 NOTE GUARANTEES. Each of International, Investments and Network
hereby provides a Note Guarantee of payment of the Securities pursuant to
Article Thirteen of the Subordinated Indenture, and it is acknowledged that
Security Holdings has ceased to be liable on its Note Guarantee because it is no
longer a Subsidiary of the Issuer.

     SECTION 2.3 REFERENCES TO GUARANTORS. Any reference in any Section of the
Indenture to the Guarantors or any of them shall be deemed to include
International, Investments and Network and exclude Security Holdings.


                                       2
<PAGE>

                                  ARTICLE III

                            MISCELLANEOUS PROVISIONS

     SECTION 3.1 RATIFICATION OF SUBORDINATED INDENTURE. Except as modified or
amended hereby, the Indenture continues in full force and effect and is in all
respects confirmed and preserved.

     SECTION 3.2 GOVERNING LAW. THIS SUPPLEMENTAL INDENTURE SHALL BE DEEMED TO
BE A CONTRACT UNDER THE LAWS OF THE STATE OF NEW YORK, AND FOR ALL PURPOSES
SHALL BE CONSTRUED IN ACCORDANCE WITH THE INTERNAL LAWS OF SUCH STATE, EXCEPT AS
MAY OTHERWISE BE REQUIRED BY MANDATORY PROVISIONS OF LAW. THIS SUPPLEMENTAL
INDENTURE IS SUBJECT TO THE PROVISIONS OF THE TRUST INDENTURE ACT AND SHALL, TO
THE EXTENT APPLICABLE, BE GOVERNED BY SUCH PROVISIONS.

     SECTION 3.3 COUNTERPARTS. This Supplemental Indenture may be executed in
any number of counterparts, each of which shall be an original, but all such
counterparts shall together constitute but one and the same instrument.

     SECTION 3.4 EFFECT OF HEADINGS. The Article and Section headings herein are
for convenience only and shall not affect the construction hereof.

                  [Remainder of page purposefully left blank.]


                                       3
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     IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture to be duly executed, all as of the day and year first written above.

                                        PROTECTION ONE ALARM MONITORING, INC.,
                                        as Issuer


                                        By:   /s/ Annette Beck
                                           ------------------------------------
                                              Annette Beck
                                              President


                                        PROTECTION ONE, INC., as Parent Company


                                        By:   /s/ Annette Beck
                                           ------------------------------------
                                              Annette Beck
                                              President


                                        PROTECTION ONE INTERNATIONAL, INC.


                                        By:   /s/ John E. Mack  III
                                           ------------------------------------
                                              John E. Mack, III
                                              President


                                        PROTECTION ONE INVESTMENTS, INC.


                                        By:   /s/ John E. Mack  III
                                           ------------------------------------
                                              John E. Mack, III
                                              President


                                        NETWORK MULTI-FAMILY SECURITY
                                        CORPORATION


                                        By:   /s/ Steven V. Williams
                                           ------------------------------------
                                              Steven V. Williams
                                              President


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                                        STATE STREET BANK AND TRUST COMPANY,
                                        as Trustee


                                        By:   /s/ Andrew M. Sinasky
                                           ------------------------------------
                                        Name:     Andrew M. Sinasky
                                             ----------------------------------
                                        Title:    Assistant Vice President
                                              ---------------------------------