3 Month Old Puppy

Okay, so here's the deal. I have a labrador/ golden retriever/ pittbull mix puppy. I am only 15 and I live with my parents (of course), so I left my dads house and I went to my moms house. The puppy stays at my fathers house. When I was leaving a bunch of my cousins had just gotten there. So while I was at my moms house I got a phone call from my dad saying he was getting rid of the puppy because it had bitten my 12 year old cousin. At the time it had bitten my cousin the kids were on the trampoline and he had tried to jump onto the trampoline to play with the kids so they tried to push him off and the puppy bit my cousin. He went to the hospital and ended up with 3 stitches. KEEP IN MIND THE PUPPY IS 3 MONTHS OLD. I tried telling my dad that the puppy had but him out of playfulness and he doesn't know what he was doing. That the puppy had to be trained. SO, I need some advice. Do you think the puppy playfully bit my cousin or do you think this dog it just crazily outrageous and needs to be brought to the pound and put to sleep?? Help please.