That’s a pretty good goal imo. I can help you with filling out the rest of your 3* roster.
Congrats on getting Bauchan! He will be a nice hero to give levels for those 3* raid tournaments. Run him as tank in your defense team for those, specially when blue isn’t allowed. Great defensive stats for his rarity, and his special attack gives his team an awesome advantage.
I would finish leveling Ulmer since he has one of the highest attack stats in his rarity. Hang on to your Gunnar; you don’t have a huge need for him right now since you have Bauchan, but still keep him around as eventually he’s a great defensive/support hero to level, when you have fleshed out your blue bench more.
Melia is one of my favorite yellow 3*s, arguably the best one tied with Bane (if not, she’s 2nd best). Her critical chance makes tile damage hit fairly hard, and thanks to her fast speed that means she’ll be ready early on through the match while you still have tiles of the strong color left.
Balthazar is great but Renfeld is meh. I’d suggest hanging on to one Gill-Ra and level her once the rest of your stronger purple bench is satisfactory for you. Gill-Ra is slow, but she is also the only cleanser in the 3* tier. She drops defense and attack for the enemies too, which makes her very valuable for raid tournaments and challenge events despite her speed. One of her is enough imo. I use her for challenge events too, mainly for the Teltoc one where yellow is a heavy enemy color.
I’d say for fodder: Ulmer 2 isn’t needed. Gill-Ra 2 isn’t needed either. Azar I’m not fond of really. Karil is okay but there’s much better blue 3*s.