Its kind of hard to know if 3 is ok based purely on age. I have some 1 year olds that are taking two mice per week, and a couple of 2 year olds that get the same.

With my snakes that get frozen thawed rats, I have based the size of the rat on roughly 10% of the weight of the snake. So a 600 gram snake would get a rat that is 50-60 grams.

However, your snake will do fine on 2 mice per week.

Steve