How long can I put triple antibiotic on a popped pus pimple? I am also on oral antibiotics. There is a red bump on my lower back and it itches after 3 days.

Need more info. This depends on the organism responsible for the pus pimple. Once it has healed over and there is no open area there is no need for an antibiotic cream (which probably does not help much anyway). These lesions can itch when healing.
As long as you like. There is no time limit on applying triple antibiotic. Your pimple should clear up in a few days at which point you will no longer need the ointment but you should probably finish out your orals.
Cephalexin. This may not be a good antibiotic if the pustule is due to mrsa. If you are not allergic to sulfa then Bactrim (sulfamethoxazole and trimethoprim) or septra (trimethoprim-sulfamethoxazole) may be a better choice.