The horses were first reported on the highway near Alpine Road at 4:50 a.m., according to CHP Officer James Evans.
A vehicle reportedly hit one or more of the horses shortly thereafter, followed by a second vehicle that hit one of the horses that was down in the slow lane, Evans said.
The second vehicle overturned after hitting a horse. One vehicle occupant complained of pain and was taken to Stanford Hospital, Evans said.
A Sig-alert was issued at 5:46 a.m., shutting down northbound traffic until the alert was canceled just before 7 a.m., Evans said.
County animal control officials responded to the scene and the horses were towed off the road, Evans said.
Although three horses were killed on the roadway, a fourth horse was found alive down the road, apparently unharmed, Evans said.
Evans said authorities are trying to track down the horses' owner.