If 2 elves agree that a workshop has lost power, it has and we are done.
If no elf says a workshop has lost power, then they are all telling the truth.
In this case, for the third trip, send all 3 elves to workshop #4 for a majority vote.
If the elves disagree about 2 different workshops, the elf that visited both has lied twice.
In this case, for the third trip, send a truthful elf to visit workshop #4.
The hardest case is when the elves only disagree about one workshop, say workshop #1.
In this case, send the elf who has not yet visited workshop #1 to visit it,
and send the other two elves to workshop #4 and workshop #5.
The answer from the first elf will tell you which answer from the other two you can trust.