Put the crawfish on a huge cookie sheet (or 2) in a 250F or so oven. They should take 10 minutes or so to warm. When they are near hot, saute the shrimp in a frypan till half cooked, which is about 2 minutes. Put in a half cup of white wine - could be stock or water - put in the mussels and put the lid on the pan, medium heat. When the mussels open (3 minutes, 4 max), pour everything on a platter and drizzle the pan juice over it all.
I will add, for the shrimp questions. Yes, 2,3,4 minutes. Shrimp, mussels and clams cook in a heartbeat. When the mussels open, they are done. When the shrimp is pink all the way through, they are done. Don't blink or they will be overcooked.