
There you go.  COM1 and COM3 use the same IRQ, therefore you can't use
a mouse on COM3 and a modem on COM1, or vice versa.  It's a limitation
of DOS.

And in fact Windows will not see a mouse on anything other than COM1
or COM2.  Accept this fact, and either get a bus mouse, or get a new
computer.


No.  The advice above applies....