5 babies out of how many?
16"x16"x18" is too big, especially for 5 babies.
At 3 weeks, they are fine together. You want a smaller cage so it's easier to find their food. My baby jacksons struggled to find food in a cage that big. The weight seems about accurate with the weight my babies had at that age. My lighter weight ones died. I (so far) have lost 5 babies, have 7 left.
They now weigh around 3 grams at week nine. Hope you've had better luck than me!