I would let it grow with multiple stems, some may say limit stems to maybe 3 or 4, but I don't think it makes much of a difference. You mentioned that you have an arbor, as long as it is growing on the arbor and not misbehaving and growing beyond I would just let it go. You also mentioned that you have 3 varieties, I'm quite sure they will intertwine with each other, you may want to limit that some just so you know which one is which. Just prune away at what you don't want, hard to go wrong, they are very vigorous growers.