How to join m:n relationship

I have two custom entities (A *-* B). Between these two I've an native n:m relationship.
Now I need to retrieve all entities from A by a condition and I need to get also all from B and here should also be an condition.
I read several blogs and forum posts but I was not able to build such a query.
Because the conditions on B are dynamic I would like to use a QueryExpression or if not possible with a QueryExpression I could also use a FetchExpression.