We 3 sleep in a queen now, though we will probably add a twin now that DS is as big as a 3 year old.
Anyway, we started co-sleeping when DS was 2 months old. We bought this:
It's really nice, because it covers the gap between mattress and bed rail, and it has little indicator windows that let you check and make sure it's locked on both ends. Really easy to make the bed, and withstands ALOT (i.e. 30 lb. DS climbing on it from the opposite side and pulling his whole body weight against it! ). It's mostly mesh, though the little bit of solid fabric at the bottom did not escape my mom paranoia (but was never an issue).
When he was a tiny tot, he usually slept on my chest (resolved breathing issues), but he also would sleep between me and the bed rail (DH is a really oblivious sleeper). Now that he's bigger, he's in the middle, often forming a giant human H.
SO...I think if we ever had another, we would get a bigger bed, but use the same set up - just keep DS between DH and I, with the new addition on me/between me and bedrail.