Yea, pmove_fixed is a cvar that controls some physics calculations, but for your case, you want to toggle 3 cvars. You use the Set command to set some actions to a name so you're would look something like this...
set camera_on "cg_thirdperson 1; cg_thirdpersonRange 40; cg_thirdpersonvertoffset 5; echo ^7fancy camera is ^5ON; set cameratoggle vstr camera_off"
set camera_off "cg_thirdperson 0; cg_thirdpersonRange 80; cg_thirdpersonvertoffset 16; echo ^7fancy camera is ^5OFF; set cameratoggle vstr camera_on
set cameratoggle vstr camera_on"
bind mouse2 "vstr cameratoggle"
I tested this quick and it worked fine for me, you need cheats on to tweak the 3rd person commands in MP. I don't recall for SP.