before, prior, preceding

I probably think that ''before'' more formal than ''prior'' and ''preceding''. Before is very suits if we r using it for showing the time. We can use ''before'' as a preposition, adverb and conjuction.

This is best: "The patients were asked not to eat anything in the three hours preceding the test."

The other two have "misplaced modifiers" (or whatever you call them). It's not clear whether they were "asked" 3 hours before the test or were not supposed to eat less than 3 hours before the test.

There's an old Goucho Marx joke which makes the point:

Joe: I shot an elephant in my pajamas this morning.

Bob: What was he doing in your pajamas?

I would say one is exactly as good as the others.

All the patients must be quite hungry by now, but less so those in sentences one and two who could have snacked after the three hour mark.

Elegant elephants in pyjamas, mmm , big ask .

Patients were required to abstain from eating during the three hours prior to the test.

I think before means something that happened to be early
Prior is giving importance to something, and Preceding is a flow of something.