I accidentally left a bag of frozen French fries out for three hours before I realized it. I put the bag back in the freezer, but was wondering if it still safe to use next time?

Three hours doesn’t seem like much, but any prepared food left out in the ‘danger zone’ of room temperature for long is flirting with trouble. When in doubt such as this, discard the product. Frozen or fresh potatoes are quite inexpensive and easily replaceable. Even though nobody likes food waste, erring on the side of caution is always the safest path.