The first two are remnants from the NCSA times, and generally, you should be fine if you delete the first two, and stick with httpd.conf.
srm.conf :- This is the default file for the ResourceConfig directive in httpd.conf. It is processed after httpd.conf but before access.conf.
access.conf :- This is the default file for the AccessConfig directive in httpd.conf.It is processed after httpd.conf and srm.conf.
httpd.conf :-The httpd.conf file is well-commented and mostly self-explanatory.