Are you asking which 3 to keep (and get rid of the rest of them)? I wouldn't get rid of any of them right now. So much will change: injuries, byes, changes in scheme, emergence of other players around them, etc.
For now, I'd start Carson and Cohen. Mixon is starting to emerge as the lead back in Cincy. A couple more weeks of monitoring of him and I could see him becoming your RB2 soon. Can't use Crowell right now, but can't get anything for him either. Martin will return week 5, I wouldn't use him that first week but he could definitely crack into your starting lineup if he takes back over the lead role from Rodgers. Henry is one Murray injury away from being a top 10 RB, so definitely hold onto him for boom potential.
I know you think your RB situation stinks, but you've got good upside here. Take it week to week, no need to make week 8 decisions now.