This mission becomes available after completing Paleto Score Setup and can be started at Trevor's house in Sandy Shores (Trevor and Michael). Get in the vehicle and head for Franklin's location marked on your map. Not far on the trip, the game will automatically switch to Franklin, who is trailing the O'Neil brothers.
|Switching to Franklin after he calls you about the O'Neil brothers will automatically begin this mission.|
Keep chasing after them, making sure you don't shoot them or anything. Once they get onto a dirt road, they will eventually steer off a cliff to avoid an elk, crashing below. They manage to get away, but Franklin will call Trevor and say he needs air support. The game will automatically switch back, so drive to the airfield and take off in the chopper.
Head for the forest where Franklin is located and you'll switch to Michael after the short scene. Switch the sniper into thermal mode (right on the d-pad) and you'll need to find the three O'Neil brothers. If you see a person with a dog, you will know that's Franklin and Chop. The only problem is that there's a lot of wild animals out there, so you're going to have to hunt for the brothers.
One of them should be swimming around in some water towards the left, while another is near the middle-right side, walking around. When you get to the last remaining brother, you'll find him shooting RPGs at the helicopter. This will make you switch to Franklin, with Chop leading the way to the last brother.
Follow Chop as he hunts down the last guy, who should be hiding behind a rock somewhere across the river. Once Chop finds him, he will go running off, so chase after him and finally kill him. Switch back to Trevor, land the chopper to pick up Franklin and Chop, then fly back to the airfield to finish the mission.