What about dual type moves, like Blaze Kick being fire/fighting?
no, just no, that would be even worse than triple types.
imagine golem being hit by a water/grass move or the already laughable magcargo by a water/ground one.
can you picture that? good, i don't think i have to say anything more.
now, regarding triple types... again, no.
even worse than making everything unnecessarily complicated is the fact that it would break the game completely.
in many cases it's already bad enough to have double weaknesses and stealth rock. i don't really see any point in upgrading an already decent but shaky chart to triple weaknesses and resistances to go with it.
also, going back to the dual type moves, i think that if GF really wanted to implement that, they had the perfect chance to do it with b/w kyurem's signature moves.
yet, they didn't and that should speak for itself.
fixing the current chart could and should, imo, be top priority, not introducing more stuff that won't help with the most prominent issues like how impractical and redundant the poison type is right now.