It sounds to me like the truck has an aftermarket turn signal switch which requires it's own, second set of bulbs. I can think of no other reason for three wires to a side.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.