How big should a tank be if I have 3 rats?

I have 3 rats I have one full grown one who is a runt so it small and I just got a new one today and she is still very small right now my space is a 20 gallon long tank and 2 of the rats are fully grown. I have also had the runt in a 10 gallon by herself for a while now but decided to put them all together please help thanks!