The question of a Triple crown is an interesting one. For me it would have to be one of the following:
A GT that is not the Vuelta (sorry spaniards) + A monument that is not MSR (sorry, but what's the point of the first 290kms) + a WC
I dislike my own opinion however, because it gives the impression that one can only be one of the greatest cyclists if you're a climber. I think we overvalue GTs.
Yeah Froome won a couple, but in my eyes guys like Cancellara, Boonen and Sagan are far greater riders. But because he has those GTs on his palmares most people will always rank him above the three I mentioned here. Cancellara, Boonen and Sagan both mastered multiple aspects of cycling. Wether it was sprinting, cobbling, timetrials or hillier classics. Froome, apart from his one Olympic TT medal, was very much a one-trick-pony.