I want to challenge Malgron the troll in the arena, but I don't know if my pets have good enough equipment, stats etc. Here is my level 25 arena team:
https://www.sylestia.com/view/pets/?petid=5966340
https://www.sylestia.com/view/pets/?petid=5966344
https://www.sylestia.com/view/pets/?petid=5966339
Any tips on defeating Malgron would be helpful too!

A few things- Your party has a 3 tank set up, which I personally don't think is a good set up. That's just me, but I prefer a Tank, Healer, Attacker set up. Some people prefer other set ups though, it depends on you ^^
Their prof points look fine, but again, I'd suggest changing some of their elements. This is what I would do:

Bone looks good, don't change him- put him in your first slot if he isn't already. If you want, change him to Earth or Water.

Grim has the wrong relic, it helps physical damage which she doesn't use at all. I would personally change her to Air or Fire, and change her accessories if you choose Fire .Put her in your last slot.

Belona's element should be Light, I think it fits her. The assasin accessory you put on her is bad though, because it helps strength, which again she doesn't use at all.

I don't know much about the arena but triple shadow is from what I've heard the best arena set up hence why they have a triple tank team

Oh, I didn't know that. I was just purely going off my battle strategy, not off arena stuff.. I should really check out more arena guides