I am glad you are interested in guppies. They are fun and colorful little guys. Unfortunately, a 3g tank is really too small for them. In small tanks like that, they don't have enough room to get their exercise and the water gets foul really quickly. The nitrite level is high, and I am guessing the tank needs to cycle, but I suggest you try to get a 10g tank. You can get a 10g tank for like $25 at WalMart.