I have a terrible staph infection and just got a new boil after being on keflex for 3 days, does this mean it's resistant?

Maybe: If you have recurrent staph infection, it is possible that you are colonized with the bacteria or the staph was resistant to the antibiotic, ( MRSA is resistant to keflex), it may be necessary to have your doctor run a culture and sensitivity test of the bacteria.

Probably: Most boils these days are caused by mrsa, which is resistant to Keflex and related antibiotics. A culture of the pus will confirm this.