If you are going to be hiking between Leigh Canyon and the Ashton-Flagg Ranch road in the N end of GTNP, then there are occasional grizzlies in that area. I would take bear spray.
If you are going to be hiking anywhere in YNP or surrounding areas N of the Ashton-Flagg Ranch Road, then you definitely need to worry about grizzly bears. There are a lot in YNP, and they are active in the fall.
Good hikes for 3 days in YNP and surrounding areas -
- Bechler Meadows (SW)
- Two Ocean Plateau (SE)
- Fawn Pass (NW)
- Black Canyon of the Yellowstone (NE)
In GTNP, the 3-day cherry is the 35-mile Teton Crest Loop. A shorter version with less miles and few scenery sacrifices is the Paintbrush Canyon-N Fork Cascade Canyon Loop. Both of these are S of Leigh Canyon and grizzly bear risk is minimal.