When they were fighting one another, it was like they were just banging their sabers against each other, neither really gained an advantage over the other until Vaylin threw a bike onto Arcann. When they fight the player, they seem to hold their ground okay, unless they're fighting a non force user. No amount of head canon will make me understand how a non force user brought them both down.
If Thexan was still around, I think he would have been the best of the three of them.
Valkorion? I don't think we ever see one attached to his npc model? Maybe he's got to a point where he considers the lightsaber to be a crude ineffective weapon?