I've been getting all 3 fossils every day for a couple weeks. I gotta couple tips that might help.

1. Probably the best is always walking, and scaling the ENTIRE map. Even the corners of the map.
2. Winter is (in my opinion) the best way to get them on the seasonal ground, because it's white. Fall, is brown, so that's pretty difficult, and summer/springs sort of even.
3. Try talking to neighbors, because then the trees in front of you disappear, which is helpful.
4. If you honestly couldn't care less about you're town, chop down all you're trees. But that is highly not recommended if you want to have a good town rating.
5. This might help, plant many flowers, because the scars of the ground can't go in "occupied" spaces, such as flowers, trees, buildings, etc.

Hope that could help you if you aren't very lucky with fossils.

(Note: Fossil scars are very small, so don't get frustrated! It just takes time.)