Swimming technique

I am a beginning triathlete and don't know the rules of swimming in competition. Is it illegal to use a different stroke at times during a race or is freestyle the only stroke allowed? In my training I use freestyle the most but I also use other strokes to help me get in the conditioning I need, and I do drill sets as well. Triathlon training is a completely new experience for me and has been a ton of fun. Can't wait to do my first Sprint Triathlon this fall.