IMO you could quite happily keep 3 rabbits in a 6 x 3 shed as it is after all the equivalent of a hutch 2ft x 9ft! (Do you like my maths skills there 8) :lol: )
However, if you are going to have loads of other stuff using up floor space then you'd have to rethink it, and they would of course need a run attached or run time anyway.
I think as your girls were so hard to get together in the first place that Id keep them just as a pair, and you have loads of other pets dont you?