I've gotten three into a LWB Hi-top sprinter before but I wouldn't try and get 3 superbikes into anything smaller. You need to make sure the LWB van has plenty of tie down anchors in the floor and possibly some on the sides as well for ratchet straps. Take plenty of old blankets to put over the bikes to protect them from the straps and the (must ensure!) the ply lining of the van.
Alternatively you could get a van with a towball and put a single bike trailer on it and two bikes inside the van - job done.