My iPhone 5 is unlocked could I use simple mobile for it?

Just need to know if I could use the sim for simple mobile

Yes it will work, but you may not get full 3G speeds since Simple mobile, which uses T-Mobile's towers, use a different frequency for their 3G service. In most places, you will get EDGE speeds, which is comparable to dial up speeds. However, T-Mobile had stayed re-farming their network to the same band that AT&T uses, so in those areas, you will get UMTS/HSPA+ speeds. However, in order for you to use Simple Mobile, your phone has to be an iPhone not from Sprint or Verizon, and it already has to be unlocked.