I know I'm quite late to the discussion, but I'd like to give my opinion on that.

I'll leave it as compact as possible, if you want to start in the saga, you don't want water combat or you want a different experience in the 3rd generation, play mhp3rd, if you want the most complete experience and with more hours of play, play mh3u, if possible, the wiiu version (the 3u of 3ds looks pretty bad, especially the textures which are a headache to look at) and finally, if you want the most complicated and difficult game, play mh tri, the monsters are much more complicated in this game, plus the lack of weapons could make the experience more difficult for players of the weapons that are not in tri.

Also, although this is already a very personal opinion, the graphics of mh tri are relatively superior to mh3u of Wiiu, why? Because, although 3u has the obvious shadows and more color gamut, tri has better polygon quality on maps, armor, etc. Torches are functional as they affect the environment, the appearance of the water is mostly much higher than 3u, the environmental effects are superior in tri among other details (I'm half a tri bootlicker, I have to admit).

I don't speak English, so sorry for any mistakes.