I've been playing around with T3 and absolutely love what he brings to a droid team. I've got IG-88 zeta'd, Bb-8 double zeta'd and R2's number crunch zeta'd. With those three, and an HK lead I've been laying waste to NS teams. T3 is still a 3*, but the other four are 7*, however, only Bb-8 is G12. Once I get them all maxed out, including T3's two zetas, they will be a beast.
They will always go first because of Bb-8. Stealthing IG-88, T3 and HK with R2 before they go will allow them to hit their AoE's and the following basic without worrying about counterattacks. If I have T3 going first then he will debuff any taunters and then everyone can wail away on the weakest opponent. Finally, T3's Combat Upgrade zeta causes everyone's crit damage to increase by 8% for each debuff on the enemy. IG-88's special inflicts 3 buffs (ability block, defense down and target lock), HK's inflicts 5 buffs (ability block, speed down, offense down, defense down and Dot) T3's inflicts 2 buffs (crit chance down and offense down) and R2's 1 buff (burning). That's 9 different types of buffs that could possibly be inflicted and, if you time it right, they will have the chance to be applied after T3 removes any defensive buffs so that takes care of any tenacity up.
Building up Bb-8's speed and defenses will be key, as he will be the one not stealthed by R2. Attacking him should be a frustrating endeavor for the enemy. Either they miss and the whole droid team gets health and protection back or they connect and he counters along with R2 which means two more possible debuffs (tenacity down and stun). So far, Thrawn has been the most problematic opponent because he can fracture Bb-8 and then that's pretty much the game. CLS and RTJ are similar with their specials. I imagine Traya would be the same but I have yet to try them against her in arena. If they can be neutralized with ability block or stun then the team has a good chance of winning.
Speaking of Bb-8, his ID counter drops very quickly because of the specials + critical hits being landed.
I plan on building up L3-37 to see if she might work better with that team. I'd probably remove HK and have IG-88 be the lead. That way, everyone is inflicting DoTs when they attack which will increase crit damage. My big concern with including an actual tank is that bb-8 taunts (I regret applying that omega) so I'm not sure how useful L3-37 (or chopper or K2) would be for absorbing damage. Her auto-revive zeta might be worth it though.