I have 3 solutions and solution A requires built versions of the dlls from solution B and C in order to compile. it is not possible to merge it to one solution...
So far
This question has popped up in different, but related, forms. There is actually an MSDN page that covers this.
What you're looking for is a multi-solution approach akin to the Partitioned Single Solution Model for Larger Systems. Have one "everything" solution that builds everything and maintains your inter-component dependencies. This is what you build when you need to build solution A. You then have separate solutions that only include components B or C. Essentially, you'll still have 3 solutions, but you'll add the projects from solutions B and C into solution A.