Can you have 3 mice in 1 cage?
Mice as a rule are social creatures. Female mice can, and should, be kept in groups of at least three (that way if one dies, there will still be a pair to keep each other company).
Males, unfortunately, are too territorial and should be kept singly as pets.
Never keep males and females together unless you want, and are willing to care for and/or rehome, a LOT of babies.
Ensure they have a large cage to run around and explore in, as mice are generally very active, smart, and curious; and will get frustrated if kept in a tiny cage. A ten gallon tank, or wire cage of similar dimensions, is fine for three mice.