As you make your way into the highway, be on the lookout for a group located on the dirt row to the left side area of the highway. Following them, use Franklin's special ability to be able to keep on the road. Just drive slowly and after a few turns, the target will be rolling down the road after an accident. Get off and run into the buring car.
Images: <Predator1><Predator2>
Switching into Michael and Trevor, find the area with the marked yellow ring. Use your sniper rifle and set it to the infrared setting. One of the targets is located on the middle slope, another one beside the riverbank, while two of them are on the ridge, waste all of them.
Images: <Predator3> <Predator4>
Swap to Franklin and execute the third brother. Keep following Chop across the river and take him out.