I'm going to keep saying don't worry. This isn't about to become a 3-man RBBC. David Johnson is their workhorse, Drake and Edmonds each had 1 great game. Johnson is the guy defenses fear. He's the unstoppable mismatch in the passing game, and he's a good enough runner to get the most carries.
I've been saying that while the schedule had helped, the Cardinals offense is looking better because Kingsbury is getting more comfortable with playcalling, and Murray is getting more comfortable. This system is pretty good, and could be elite if Murray becomes as good as I think he can be. Any RB should be reasonably productive in it. That isn't to take anything away from Drake, Edmonds, or Johnson, they've all looked good, but as the old adage goes, if everyone looks good, maybe look elsewhere for the cause.
I look at these 3 RB's, and see 3 pretty equal rushers. Johnson obviously crushes as a pass catcher, and while I think Edmonds might actually be the best short yardage runner, I'd be shocked if Johnson didn't get that job, based on his size, history, and just having your "best" player on the field in the most important situations.
When Johnson comes back(seemingly next week) I'd probably have him ranked around RB8 or so. Drake probably around RB30, and Edmonds not ranked at all, and he seems unlikely to be back for a couple more weeks anyway.
ETA: Its kind of funny to me, I was calling David Johnson overrated for years, but I feel like now that other people are on board, they have gone too far in the other direction.