You don't necessarily need to provide a separate OCPD for each appliance as long as fault protection is OK for the tail to each appliance. Also check manufacturers instructions in case they specify a fuse size,which regardless of how pointless it seems you will have to comply with. Otherwise I would just link the incoming 6,0mm(?) between 3x 20a DP isolators,MK (for example) 20a isolators will take 2x 6.0mm conductors, because I have terminated them in the past. Or you could use 32a isolators if you prefer. 45mm deep back boxes will be a good idea, 35 will be tight....don't even think about 25mm