I doubt if you bought grafted grapevines, but maybe you did. If I remember I cut the two or three shoots back to three or even four buds.

European varieties are usually grafted for phyloxera resistance since they don't have any natural resistance. If you have American varieties they usually aren't grafted.

I looked it up. There's a "crown" that looks to me like a graft.

Anyway, I pruned above the crown to 2 buds. Assume you rub out the buds below the crown.