It runs from SR 36 north along the shore of Trinity Lake, Fort Jones and Etna.

The route then approaches Yreka, intersecting with Interstate 5 (I-5), and turns east to Montague.

This stretch of road through the Shasta-Trinity National Forest parallels the Hayfork River.

[3][4] In Siskiyou County, SR 3 passes through Callahan, Etna, Greenview, and Fort Jones as it turns northeast to intersect with Interstate 5 in Yreka.

[19] Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, based on the alignment that existed at the time, and do not necessarily reflect current mileage.