And, this should be in the Rulings Forum, not the Commander (EDH) Forum.
Now, onto the answer: if Inferno has a single target, then it has one target. It doesn't do anything 3 times and you can't target the same thing 3 times. With one target it is effectively a Banefire and Torbran will modify the damage to add 2 to it.
Jaya's Immolating Inferno
Torbran, Thane of Red Fell
And, this should be in the Rulings Forum, not the Commander (EDH) Forum.
Now, onto the answer: if Inferno has a single target, then it has one target. It doesn't do anything 3 times and you can't target the same thing 3 times. With one target it is effectively a Banefire and Torbran will modify the damage to add 2 to it.