People keep thinking of the 3 classes as very similar and all related when really they're 3 different things entirely. It's not like being a soldier and having 3 different loadouts. It's more like being a body guard, a professor/archeologist, and (you guessed it) a hunter. Titans are the insurmountable walls that defended people as the great warriors and soldiers. Warlocks are scholars so intelligent that they're brought to the brink of reality and can bend space to their will, using this as a means of protection while their primary goal is collecting and maintaining knowledge that can help defeat the darkness among other things. And the hunters were people that had to adapt to the wilderness to survive while the major cities were being destroyed. They learned how to hunt great monsters, move lightly to avoid detection, and all around survive so you know they are good at combat and mobility. It so just happens that since the universe is so heavily populated with enemies, everyone needs to learn how to survive. It's just that those three specific roles are very important (protecting others, gathering knowledge, and scouting/surviving).