Of those three, I feel that blue would be the easier color to cut if necessary, so if Tectonic Reformation is important to your strategy, I'd suggest at least WRG.
Gahiji, Honored One may incentivize your opponents to hit each other at first, but I fear this may only work short term.
Hazezon Tamar has an insane ETB, but would draw way too much hate, and there are better ways to build him without trying. He'd steal from the theme of the deck.
Kynaios and Tiro of Meletis gives you blue back, and is an insane blocker. He'll help ramp and draw you, but doesn't add anything directly.
Kydele, Chosen of Kruphix would be a rather great blink target if you had some global haste enabling, such as Anger or thousand-year elixir. If you're set on red though, you'll end up picking one of the RW guys to go with though.
Ravos + Kydele could be good, but that completely drops your Tec Reform idea.
Black and red can both add a lot, but I feel that's just choice between utilities. Archfiend of Ifnir is absolutely insane, and Shadow of the Grave can be quite potent as well. Black also gets you great support, in things like Phyrexian Reclamation to bring them back.
Of those three, I feel that blue would be the easier color to cut if necessary, so if Tectonic Reformation is important to your strategy, I'd suggest at least WRG.
In the Partner Commanders
Black and red can both add a lot, but I feel that's just choice between utilities. Archfiend of Ifnir is absolutely insane, and Shadow of the Grave can be quite potent as well. Black also gets you great support, in things like Phyrexian Reclamation to bring them back.
Retired EDH - Tibor and Lumia | [PR]Nemata |Ramirez dePietro | [C]Edric | Riku | Jenara | Lazav | Heliod | Daxos | Roon | Kozilek