Rat cages need to be at least 4 cubic feet in size with an additional 2 feet of cage space for each rat more then 2. So 3 rats would need a cage that is at least 6 cubic feet in size. A rat cage needs to have a base of at least 30 inches by 18 inches and height with levels.
I hope that you will get all the girls spayed or the boys neutered by a vet experienced in successful surgery on rats .
(See
http://www.ratshackforum.com/forum/showthread.php?t=7691 for info re surgery)
There are many health benefits to getting girls spayed.
Having 2 unaltered sexes in the same household is an accident waiting to happen.
Rats are very smart, and very motivated when someone is in heat.
And people are not 100% perfect all of the time.
Rats go into heat every 3 to 4 days.
I have heard of rats getting out of cages in the basement and getting to rats of the opposite sex in a third floor bedroom.
A double CN cage has small bar spacing, can hold up to 10 rats and can be divided into 2 cages.
It is a wise investment so you might consider getting one.
You should also be aware that it can be very difficult to find good homes for rats so you may end up keeping more rats then you were planning on. Here is some info that may help
http://www.ratshackforum.com/forum/showthread.php?t=23041
Hope this info helps.