Why'd it have to be a triple battle...? I'm still learning singles and doubles, and I have absolutely ZERO experience in triples. Time to go find some guides...I'm not expecting to do well in this format (especially since I tend to use 'mons which aren't the typically used powerhouses, with odd movesets).
We're still working on our Triples analyses, but we've sorta gotten sidetracked because of college reated matters. Thankfully though, you can transfer a lot of skills you've learned in doubles to triples. Just remember a couple of important concepts!
1. Positioning is huge in Triples! In most cases, your Pokemon can only hit opponents that are adjacent to it. I.E.: If you have a Pokemon in one of the corners, it can't hit Pokemon that are in the opposite corner. There is an exception to this rule: If your Pokemon has Flying-type moves or Aura Sphere, Dark Pulse, Dragon Pulse, or Water Pulse, you can hit Pokemon no matter where they are located!
2. You can change the position of your Pokemon with "Shifting". Pokemon in the corner can Shift instead of moving, where they switch places with the Pokemon in the middle position. How soon your two Pokemon switch positions depends on the Shifter's Speed. Interestingly enough, the shifter can conserve their Fake Out for next turn this way as well if they haven't moved yet.
3. As you can imagine, spread moves are huge in triples, and such Pokemon are usually sent in the middle to hit all 3 Pokemon. Because of this, you often see Pokemon such as Hitmontop used in triples for Wide Guarding, to block said spread moves. Note that these won't block Dark Void. In fact, a major hallmark of this particular competition is that most of the usual Wide Guarders (such as Hitmontop, Aegislash, and Hariyama) are actually banned in festive feud, which is why we've been talking about oddballs such as Regigigas and Throh: Wide Guard is that valuable!
4. Unlike other battle spot formats, you use all 6 Pokemon! This also means that you should only be carrying one mega, and oddly enough, makes triples feel more like a smogon format than usual in some ways.
5. I find that support Pokemon tend to be a lot more justifiable in triples than other metagames, since you got all 6 Pokemon and have two other Pokemon that can deal some serious damage.
Of course, this isn't everything, but I hope these pointers can help you a bit
mdchan !