With the lack of signage for vehicle three it has the right of way for several reasons, including state law in my state. The car on the right has right of way. They are going straight down the primary road too.
I have long had the opinion that the law should be the opposite, conforming to most practices, the person on the left goes first. A lifelong thing with me, if I come to an intersection at the same time as another vehicle I always have the vehicle on the left go first for the simple reason I can see the driver with fewest obstructions.
Strangest intersection I have ever seen in the US, I think it was in Pennsylvania. A decent sized side road entered a four lane on the side of a substantial hill and there was a traffic light there. All pretty routine until I saw the sign on the side road, "Yield on Green Light". Best I recall it was because of heavily loaded trucks coming down the hill, they probably couldn't stop anyway! The only time I have seen a solid green light not give you right of way on a highway. Might have been for heavily loaded trucks going up the hill too. It was steep enough to twist driveshafts or axles trying to climb it from a standing start.
Hu