If you’re going to use a three-gallon tank that’s been treated with fenbendazole as a micro reef with live corals, the safest thing to do would be to replace the substrate and live rocks. That should not be too difficult or expensive for such a small aquarium.
Otherwise, you could try continuous filtration with a good grade of activated carbon to continuously remove any trace amounts of the fenbendazole (brand name Panacur).
Best of luck with your micro-reef project, Taylor!
Pete Giwojna, Ocean Rider Tech Support