Out of the three, Darmanitan is your best option, with Arcanine in second. Darmanitan is an absolutely brutal physical attacker, with access to Sheer Force and moves like Flare Blitz. It can completely pummel most opponents, and its frailty doesn't much matter in-game. Arcanine is also extremely good, as it's a physical Fire type that you already have. I don't like it as much as Darmanitan, as Darmanitan has always worked better for me, but it may just be a matter of preference.
I don't think you should use Volcarona because it doesn't get very many good moves until quite late in the game, meaning it can be deadweight for quite a while after you get it. You also already have an abundance of special attackers on your team. Better Bug types for in-game purposes are Galvantula and Scolipede.
Darmanitan @ anything
Ability: Sheer Force
- Fire Punch / Flare Blitz
- Headbutt / Belly Drum
- Zen Headbutt / any other physical move
Hope I helped!