leave the norco and xanax out. The norco will make you puke your guts up. The grapefruit juice potentiation is something that happens over time in the liver. You gotta be drinking GFJ all day for the enzyme to work its magic then take the dxm. It will also potentiate the xanax. Maybe the xanax will keep you calmer on the DXM but personally i'd leave it out as I don't see much benefit from mixing them in there.
This is a very dangerous combo in my opinion, even though the narcotic doses are low it's almost a guaranteed blackout, and mixing these three substances in general can be fatal if you screw up the doses.