Looks like you’re getting some great advice here 
And your alliance mates gave you very good recommendations too.
You want to level a team of rainbow 3*s first ideally so that you can compete in events and finish as many quests as possible. 3*s are very quick to max and have the best quickest available stats, so that’s why they’re recommended first. They’re also allowed in every rarity tier, while the higher rarity heroes (while better) are not allowed in rare events and tournaments.
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You are correct – costumes give a very nice stat bump to your heroes.
With that in mind;
Yes, personally I would say so. Jahangir is decent but he’s very outclassed now compared to the newer 3*s that have been released. I would definetely do Hawkmoon + costume first.
Isshtak is a great hero, I think you chose correctly to level him. With costume he becomes a force to be reckoned with, his defense debuff is high. I use mine without costume personally for the higher damage, as I have other defense debuffers that I prefer over him in 3* rarity – so both forms are useful
You should work on Gunnar for blue next, he’s one of the best 3* in the game with his costume. Costume ability forces enemies to share damage while getting reduced defense, that means you can target the weakest link in the enemy team (the one with the lowest defense) and hammer them with special skills to spread a lot of damage amongst the enemy team. He is an extremely useful hero, I even bring costumed Gunnar in Legendary challenge events! 
Kailani is like Gunnar expect she has no costume currently, so her usefulness is much lower than his. I would still max one of her personally, she is helpful early on to boost your survivability. I actually completed the entire Season 1 campaign without a single maxed 5*, all thanks to Kailani as she was part of one of my key teams. So yeah I’d say she’s worth maxing, she will eventually be benched but she’s useful while you are this current level.
You are correct that Tyrum is weak with his hit, but his main purpose is more so to provide support for your team. With that in mind he has nice survivability for a 3* and his main appeal is dispelling enemy buffs, which is very useful.
I would level Renfeld + costume eventually as well, and only use him costumed (default is rubbish). As Wizard said he’s a great asset against bosses with costume.
