So they don't just have one litter each then, I'm sure he will too. Forgot to ask: My three girls who are going out with Buster are inside, what month do you think they will be ready to go outside in? They were born the end of July 2011
It depends on whereabouts you live and what the weather is like also what kind of accommodation are they moving into.
The further south in the UK the sooner you could move them as an example spring in Kent will be around the end of march beginning of April while here in Northumberland it is end of April beginning of may.
Should the weather forecast be frost free then by all means let them have the space to explore making sure you have adequate protection for them in the way of warm dry beds away from the prevailing wind.
If like some you provide a shed as part of the living accommodation then you can advance the moving time by another two weeks in my opinion making a move around mid February.
Is Buster currently outside?
If he is you may find your girls calling for him sooner than February which is ideal as they are more likely to accept being moved if they are looking for a mate. Should they start calling by the end of January however I would advise waiting 2 weeks until they call again just to be sure.
Hope I haven't put too many dates in but it is a weather based and personal decision which only you can make when the time comes.
Buster however will not complain if he has 3 willing friends moving in, He will probably offer them all space to keep warm