Don't use only 4 holes to mount 3 seat belts!
They are designed to keep you from flying out the windshield only when each end of each belt has it's own hole and large washer under it.
By doubling up you double the load on the center two attachment points when 3 people are in the truck in a wreck.
Not a good idea!
I am not trying to say I don't know someone who has done it that way, as a mater of fact, I know him very well, but it's still a bad idea.
Grigg