The Three Brothers Trail is part of the longer Heather Trail (44km total, usually hiked as a multi-day). It is a rewarding 20km day hike with optional summit opportunities.
If you don't have the time or energy for the full hike, a shorter 16km version is still exceptionally rewarding, with views of surrounding mountains and alpine meadows filled with wildflowers.
With the trailhead being located at 1980m, there are scenic views right from the parking lot. The trail then heads down to Buckhorn Campground (reservation required) before returning to the sub-alpine. Total elevation gain is around 800m.