Learn Dutch: Daarvoor moet je het paneel ontgrendelen. - For that you must unlock the panel.

Because it is at the very beginning of the sentence. When you move a pronominal adverb (a combination like ervoor) to the first position to emphasize it, the er- part must change to daar-. You generally cannot start a sentence with the unstressed ervoor.

If I wanted to say "For that you must unlock three of them", does that trigger the double er rule?

Yes! "Three of them" requires the quantitative er. If you tried to put both that and the prepositional ervoor in the middle field, you'd create an awkward double er. Because Dutch actively avoids this, using daarvoor instead is the perfect substitution strategy. You simply say: Daarvoor moet je er drie ontgrendelen.

If I didn't start the sentence with "for that", where would it go and how would it change?

If you start with the subject je, the adverb moves into the middle field. Unless you still want to strongly emphasize it as daarvoor, it changes back to the unstressed ervoor. A standard, neutral word order would be: Je moet het paneel ervoor ontgrendelen.

How is the verb ontgrendelen built up?