Hi Naomi,
i have bred in trios before but have only occasionally had it go smoothly!
I usually find that one of the females will get dominant and pick on the other female so they have to be split up.
I have also found that sometimes when both females have litters together they end up arguing over the babies. I have seen two females have two nests at opposite ends of the cage and they keep trying to move the babies to their nest - so they will both be trying to move the babies to opposite ends of the cage at the same time. Stressful and dangerous to the babies and stressful to the parents too.
Sometimes i have had two females nurse their litters together but this has been quite rare for me and usually when it is a female and her daughter rearing their litters together. I find that females tend to get along better without a male there.
I prefer to keep one male to one female - much easier in my opinion
Hope this helps,
Tammy x