Zarya Overwatch's 3 vs. 3 elimination mode, out since November, turns on its head everything we know about team composition. Heroes beloved in 6 vs. 6 Quick Play and Competitive modes, like Lucio and Torbjorn, are damn near useless in 3 vs. 3, where your priorities are a bit different.
That makes composing a viable team a bit sticky. On Ecopoint: Antarctica, the home of 3v3 mode, your team will want to stay together, making heroes who rely on flanking pretty risky. Heroes with high health and low damage output are just charge fodder for an enemy's ultimate ability. And for Bastion, well.... don't do that. You're a sitting duck. Stop it.
There are a few popular strategies for besting 3v3 mode: Splash/burst damage, splash/burst heals and getting the enemy team down to two asap. And there are a few team compositions that maximise those things. Here are my suggestions for team comps that keep everyone as alive and deadly as possible:
Roadhog, Soldier 76, Ana
Roadhog is a must-pick for 3v3 mode. It's simple: His hook is devastating. Plucking off an enemy first heavily weighs the game in your team's favour, and Roadhog's large enough to absorb damage up front, so he can lead the push. And if his hook lands (through a wall, for example), that's a quick kill. His scrap gun does rad splash damage up close and he can heal himself, so he's basically a perfect defence, offence and heal character wrapped into one big, wonderful porcine dude.
Soldier 76 can also heal himself (and his whole team) with his biotic field, but his real contribution to the team is his rifle's powerful medium-range damage. His Helix Rockets do a ton of splash damage, so if the enemy team is sticking together, they all risk getting hit.
To round out the short/medium/long-range damage triangle, Ana is your best bet. Her splash heals are incredibly effective for healing the team. And, by targeting one enemy with her sleep dart, she can separate them from their team, leaving them open for Soldier or Roadhog to finish the deed.
Roadhog, Pharah, Mercy
This team is basically invincible. Pharah's a killer initiator and can knock out an enemy or two with just a few well-aimed rockets from above. With Mercy damage-boosting her, she'll knock out enemies that much faster.
Roadhog here is best flanking enemies who retreat from the central area to avoid Pharah's rockets. Since he can heal himself, he won't need too much attention from Mercy. And, if he times it right, he and Pharah can land some nasty dual splash damage with his gun and her rockets.
Reinhardt, Soldier 76, Ana
Reinhardt does insane damage with his hammer and charge attacks in Ecopoint: Antarctica's tight hallways. And he can shield the squishier Soldier and Ana from attacks. This team does well to stick as close together as possible, though, since Soldier's Biotic Field and Ana's healing grenade have a short range, but can keep Reinhardt alive forever.
This team's a bit more defensive. Although Soldier's damage output is crazy, he'd be best to stick behind Reinhardt's shield. In that sense, it might be best to wait for the enemy to come to you rather than actively pursuing them.
Roadhog, McCree, Zenyatta:
This is a very active team. Zenyatta's Orb of Discord will help McCree and Roadhog take out enemies quickly and without too much effort. And his Orb of Harmony will keep his team alive while McCree and Roadhog push forward into enemy territory. If Roadhog is in top tanking form, he'll be absorbing as much damage as possible while McCree and Zenyatta lob longer and medium-range attacks at enemies.
Reinhardt, Mei and Roadhog
This team is invincible. With Mei and Roadhog's self-healing and Reinhardt's shield, this team can outlive enemies without a support hero. Where Mei shines is separating enemies from each other. She can raise an ice wall between an enemy team so Roadhog can hook them or Reinhardt can hammer them down. And if she sneaks up behind enemies and freezes them, that's another easy kill for either of the two tanks.
These are my picks, and they depend heavily on healing, a variety of attack ranges and splash damage and heals. Add your favourite 3v3 comps in the comments!