From the ones you listed there - undoubtedly Balthazar. Renfeld is not so bad either for titan hits since he has high attack and decent hp value. Oberon and Prisca are food. An-Windr is a disappointment.
Thanks!! I was so excited about An-windr but he really disappointed me!!
Best 3* purples are Bjorn, Guardian Bat, Balthazar and costume, Tyrum and costume, Gil-Ra, Chochin, Jack. There may still be others but those aforementioned are the ones on the top of my head.
I would also include costume Renfeld, although the base version is trash. And yes, An-Windr has been terrible when I’ve tried him out.
You like the costume renfeld better?
Yes. He basically has Onatel’s skill as a three-star, and Onatel is still my favorite hero on my entire roster. Mana control is king. C. Renfeld pairs well with Chochin.
He’s got Onatel’s special with costume, so you can get him almost constantly firing. Doesn’t do enough damage in base form.
Jinx. (plus 20 characters)
Renfeld and costume. His costumed version does have Onatel’s mana generation stealing. The problem is, Onatel damages and affects target and nearby while costumed Renfeld only affects the target. Thus, I’d rather have a 3* purple sniper that deals huge damage against fellow 3* heroes, which has less defense and health and easy to kill.
Read Renfeld’s skill again!
Is Balthazar or costumed Balthazar better??
If looking for sheer damage the base version, if you need more survivability the costume.
Damage only affects the target, but mana generation stelling affects target and nearby.
For me, this is the best non-sniper 3* purple.
I level one of each 3 star as even the slow ones are great during rush tournaments and during challenge events where you can mana them up.
Costume is always worth leveling since it provides stat bonuses. Afterwards you can decide on which version you like better. I don’t see you have Balthazar’s costume yet though so you don’t have to make that choice yet.
For me, as I said, Renfeld even non-costume (which you have) is still somewhat worth leveling up because he can help versus yellow titans with the best tile damage amongst the classic 3* purple heroes. His attack value is even higher than Balthazar’s and he is more durable vs titans.
Since you don’t have many good purple options, I’d suggest you start leveling Balthazar first, and when you max him, if you pulled/trained nothing better in the meantime, go with Renfeld.
i have the costume for Balthazar, so you suggest leveling the costume version rightÉ
You can do both versions, but with the costume leveling up trick costume version is faster to bring to level 50. Just ascend the original hero to the last tick, level 1, then put on the costume and level that to max. You won’t be able to do it unless you first ascended the non-costume version of Balthazar to max. Afterwards you can focus on Renfeld, and when you’re done, you could return to Balthazar to max out the original hero as well.
May I ask where you use Guardian Bat? He was my first Guardian, and I rarely use him. 
In both offense and defense in raid tourneys and rare tier in special events, if the occasion rises. I had two Bats both at +15. Stripped both of them so that I can fully emblem in all sword path my Alfrike, who is the 2nd Wizard class hero I have fully emblemed (the other one is Onatel). On defense, I placed my 2 Guardian Bats at wings, who is like mini mini Guardian Owl that only hits the target with minor damage to nearby minus the defense buff and heal over time. Now, one of my Bats is back at +15 while the other is still without emblems.