The 3of1 v 1of3 argument is just 2 sides of the same coin. 3 warlocks? Ok you'll hit level cap sooner but have no use for drops of exotic gear etc for the 2 classes you don't have. It could be argued that you deny yourself 2/3 the playing experience too but that's debatable. 1 of each class? Ranking up is more tiresome because of weekly loot limit on NF and raids. You can't swap armour to powerlevel with bounties and raid drops can shaft you, but you get the full spectrum of options and gear usage. I have 1 of each just because I felt like trying each out, but really it shouldn't matter to anyone what route you choose to play