Flames of the Firebrand deals 3 damage divided as you choose among one, two, or three targets.
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.)