When the third ability resolves, you must change the target if able. If you can't choose a legal target for the spell when this ability resolves, that spell's target doesn't change.

If a spell has multiple targets, but it's targeting Torchling with all of them (such as Seeds of Strength targeting Torchling three times), you can target that spell with Torchling's third ability. In that case, you change all of those targets to the same new target, as long as it meets each of the spell's targeting conditions.

If a spell targets multiple things, you can't target it with Torchling's third ability, even if all but one of those targets has become illegal.