Zekrom, along with its counterpart Reshiram, came out of the Gen 5 Movepool lottery in good shape. Being stuck with Dragon Breath hurts its Dragon-type performance compared to the plethora of Dragon Tail/Outrage Dragons; however, Zekrom received pretty much the best Electric-type moveset it could have hoped for in Charge Beam/Wild Charge. Backed by a massive Base 275 Attack stat, Zekrom is the undisputed #1 Electric-type, with significantly higher DPS than Electivire and bulk higher than Raikou.
The Zekrom trio is, to be blunt, easy. The fact that the Zekrom duo is even possible, albeit extremely hard, means that 3-manning it will be quite easy; even against Outrage, Level 30 counters are strong enough to win comfortably with 3 people.
Minimum viable levels assume Best Friends, no Weather, no Dodging.
Italics indicate a Legacy move.
Use Dragons; if you don't have Dragons, use Ice-types; if you don't have Ice-types, use Fairies. To be frank, trioing Zekrom is really easy, and the only thing you really need to pay attention to is that you have the right moves on your Pokemon.
Flash Cannon, if not dodged, hits Ice-types and Fairy-types very hard. While Mamoswine, Glaceon, Gardevoir, and Togekiss can still comfortably trio, dodging Flash Cannons will improve their performance by a fairly significant amount.
Weavile needs to be at Level 40 to trio against Flash Cannon; alternatively, Level 30 Weavile can be trio-viable against Flash Cannon if you can dodge all Flash Cannons.
Wild Charge is a little harder than Crunch or Flash Cannon, but the trio is still so easy that as long as you have the right movesets and your Pokemon are at the specified levels or higher, you'll win with plenty of time to spare.
Without dodging, sub-Level 40 Salamence and Rayquaza are non-viable due to how often they will die to Outrage at lower levels.
If you can consistently dodge all Outrages, Salamence and Rayquaza become trio-viable at Level 25.