I can only level one up once they get to the 3rd level
Which one would you go for?
All good heroes. Can you give us a sense of what other heroes you have already leveled-up? Are you looking to improve your ability to raid? War? Titans?
It depends on what other heroes you have already. If you have and use minion makers (telluria, Freyja, Noor, etc) then Frosth is a good support hero for them. If you need a sniper type then Uraeus would be my choice. Yang Mai looks useful if you need red.
And don’t forget to factor in what emblems you have saved up … that can help you choose.