I saw plenty of huge female guppies at my lfs today. I didnt like them, very ugly..lol Ive heard warmer water helps them grow faster, I assume that they will all grow that big with time?
Please don't try to grow your guppy to be 3+ inches. It's not natural. They're supposed to be 1-2 1/2 inches maximum. 3 inches is just plain wrong. I'm breeding guppies, and just because I'm talking on here, I'll offer: do you want to buy some baby guppies? I will be selling them when they are at 3 weeks old (so I can tell gender) and I'm thinking of a price (if any) to be around $1.00. I will not sell you sick/parasitic/fungal fish (and if you or I find a sickness in the babies I will fix it as soon as possible!)