I tried keeping multiple moms in the same tub as they give birth. You'll lose half the babies from them 'baby hoarding' and end up with as many as if you had 1 mom alone - and you'll have to clean out dead pinkies daily. Not pleasant.

Find a way to keep the moms alone during birth and until the babies grow fur.

With 6 tubs, I'd keep 3 females per tub in 4 tubs and rotate a male every 3 weeks between them. As the moms get ready to give birth separate them 1 in the original tub and 1 in each of the 2 empty tubs. By the time the next tub has hit 3 weeks you can put the moms with all the babies back in the original tub. This gives you room to take the next tub and split it - 1 mom in the original tub and 2 in the empty 2. After 3 weeks put them all back etc.

4 tubs - 3 females each. 1 male - 3 weeks per tub. As you move the male, split the 3 females from the previous tub and put the 3 separate females back in their own original tub.

You're welcome