Working with two different set of colleagues on the same problem
Actually, you should let A know that you and the others have been working on the problem and propose that you merge into one group. If B and C haven't been productive on it, then you might want to discuss it with them first. It might be that one or both of them isn't ready for a more intensive collaboration.
You can, jointly, be honest with A that your progress has been slow.
But it would be a mistake to arbitrarily exclude anyone.