Whether or not he runs the three will be entirely up to NASCAR and whatever owner is given that number.
My understanding has been that if an owner doesn't use an assigned number that after a certain length of time (thought it was three years) that number will be reassigned to whatever owner requests it.
I'm not sure how RCR has retained that number this long as they have not used it on a Cup car since February 2001 that I'm aware of.
Jr. will have no say in whether or not he drives the 3 car, unless he either owns his own team or makes arrangements with whatever owner he happens to be driving for to acquire the use of that number.
As for multi-car team, RCR already is a multi-car team and it doesn't seem to be doing him a great deal of good, does it? If that were the case, why wouldn't he simply expand his own operation instead of investing in another which has been a lot less than stellar in the Cup series of late?
What happened to RAD, the three team conglomerate that was supposed to "rule" NASCAR a few years ago? That idea didn't seem to work out very well, did it?