Out of the three, Darmanitan is your best option, with Arcanine in second. Darmanitan is an efficient physical attacker with access to Sheer Force and moves like Flare Blitz. Its tendency to OHKO targets makes its low defenses less of a hindrance than they appear. Arcanine is also reliable, as it's a physical Fire type that you already have. I don't like it as much as Darmanitan, as Darmanitan sacrifices unnecessary bulk for a damage increase, 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 late in the game, making it dead weight for most of your run. 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
- U-turn
- Zen Headbutt / any other physical move
Hope I helped!