For feeding three snakes I would say you would need about 5.10 mice. That would give you about 80-100 babies a month. If you have a bad month with breeding you can freeze em as they grow up.

Cost of a mouse rack is cheap! I built my whole rack for 25 dollars and 14 of it was for the hardware cloth. It's a 15 tub rack using dollar store bins.

With only 3 snakes mice will take up much less space then rats and be much more manageable.