Of course, there are some challenges. We have a queen (no room for king or sidecar crib) and while DH and I are not large people, it's certainly tighter with 3. The order is DS, me, DH. I finally removed the rarely-used co-sleeper and put up a bed rail. This has helped somewhat as we can sleep closer to the edge, but DS and I always wind up towards the middle of the bed. He doesn't even move much, just wraps his little arms around me and sleeps. I think it's due to gravity.
Thing is, when I switch up the order so I'm near the rail we have so much more room as I stay really close to the edge. I do this to feed him from my other boob as we like to do side-line at night. I don't roll towards the middle like DS does and it feels so much more spacious. While I try to keep myself awake during the feeding so I can switch us back, I've dozed off now and then. My concern is that it's not supposed to be safe for dads to sleep next to baby. DH has also expressed interest in sleeping next to our son and insists he's always aware of him, but I tell him no, it's dangerous.
Does anyone let their LO sleep in the middle? If so, what age did you start?