Congrats on your babies!
Were your litters born on the same day? If they were born within a few days of each other you can give 3 to the foster doe since she has a small litter. One thing you need to consider first is the temperament of the foster doe, is she happy to let you touch her kits? Some does make great foster mothers, some just won't have it even if you get the kits in there as soon as both litters are born. As long as she doesn't mind you being around the nest she should be fine. I have used surrogate does about 7 times, most often in this circumstance where you have a large litter and asmall one,and only had one doe who refused to feed the foster babies, most will take on extra kits and love them like they're their own!
Gently rub some of the nest fur from the surrogates nest on the kits to hide the smell and then rub it on the doe too. Keep a close eye on them just to make sure the doe is settled and happy. If the doe is feeding the 3 fat ones the other 3 should be getting fed too, but if the other 3 have had a head start the smaller ones may find it difficult, and since the other doe only has 2 to feed it would be benefit the kits and the doe with 6 to feed to spread the kits out.
