is mando viable with 3 stars/1zeta and gear level 11?

He can certainly be viable, you just have less options as far as teams you can face or team comps you can use. You'll pretty much 100% need Bossk on the team to taunt and take the heat off of him. Another thing you can do is use a Boba lead, which will give you a ton more health provided your Boba is modded right and high level. His lead gives max health to all Bounty Hunters based on potency, and the max health number is based on Boba's health pool, not each individual BH so you could potentially get 100k or so max health added to everyone