Wow...3 girls in a 5 gallon really is just a ticking time bomb waiting to go off. But I've already explained all of that....I won't go on about it. xD
Anyway, if you just have three, you won't need any more tanks thankfully. Just keep your one girl in the 2.5(and divide it for now if you need to)then make a craft mesh divider for two of them permanently in the 5 gallon.
Here is a great tutorial: http://www.tropicalfishkeeping.com/d...ividers-21866/
When I make mine, I use two sheets of mesh and double up, then do a little more precise measuring/trimming to make it fit snugly but still securely. This seems to work best at keeping the dividers straight and stable.
Also with two girls, you'll want to drop the water level a tad bit and raise the divider up as close to the hood as you possibly can since girls are excellent jumpers with their short little fins.