At 2 and a half weeks old, your bunnies should be starting to venture out of the nestbox. When it gets to the point where they keep getting out and won't stay in there anymore, you clean the nest box out completely. Then you can turn it on it's side and put a pile of hay in the back corner, so that they can still all snuggle together in there. If they aren't trying to get out yet, you should clean it out. Try and keep as much of the clean dry fur as you can, then put fresh hay in there and hollow out an area at the back, and put the fur back in that hollow.