You divide the damage as you cast Flames of the Firebrand, not as it resolves. Each target must be assigned at least 1 damage. (In other words, as you put the spell on the stack, you choose whether to have it deal 3 damage to a single target, 2 damage to one target and 1 damage to another target, or 1 damage to each of three targets.)

You can’t deal damage to both a player and a planeswalker controlled by that player with Flames of the Firebrand.