Three of the same class or three of different classes

So I read somewhere that the best way to farm mats (etheric) and to have the highest ranked up gear is to have the same class, and I ran with it. After talking with a clan mate, I think I may have done so in error. :( For instance, if someone needs a titan in a raid and I'm stuck with my three warlocks.. :( Or if you buy an exotic helm emgram and it pop for another class, you may still be able to use it on another character... Thoughts?