Let's bring out a new topic, shall we?
Which combination of the strongest Legendary Pokemon do you prefer to have in battle together (Kind of a Trio question, only with the strongest) and explain?
Mewtwo, Lugia & Ho-Oh
Groudon, Kyogre & Rayquaza
Dialga, Palkia & Giratina(Alter or Origin Form)
Reshiram, Zekrom & Kyurem
That's four groups, all with base stats of 660-680.
Personally, I would stick with the 4th Generation. Dialga and Palkia don't have very many weaknesses for one, and Giratina's Origin Form makes it a complete utter disaster. Plus their movepools are just real great, many to choose from.
But I had a gut saying Mewtwo/Lugia/Ho-Oh at first. Stick with the originals. Lugia and Mewtwo are just deadly all around as beasts, but Ho-Oh's 4x Weakness to Rock hurts this group. 5th Generation Dragons of Zekrom and Reshiram are also great Pokemon, with Kyurem a little weaker. But I don't want to explain or even say that the fusion with 2 of each can play a role and I'm not sure how that is quite yet. So 3rd Generation of Groudon, Kyogre and Rayquaza are by far probably the worse of the gang. Not to mention their abilities will cancel each other out way to much.