Well, the only way this can be anywhere near close to even thinking about would be if just a few of the Pokemon had three types. I mean, right now, there are Pokemon with one and two types. Just because a third slot for a type could be introduced, doesn't mean that every Pokemon has to have three types. Just like Magnemite and Magneton were changed to dual Electric/Steel, albeit a different reason, you could change like three or four pokemon to three types (namely Charizard, Gyarados, and Aerodactyl) to add the Dragon type in, because come on, we all see it in them. There might be a couple more out there that deserve a third type, and sure, give it to them, and introduce a few more that have a third type. This doesn't mean that suddenly every Pokemon has moves that are super super super effective against it. If they were to introduce say 147 new Pokemon (to make an even 650) in Gen V, then only have like 15 of them be triple type.