Looked it up last night, the three must be at least 6 inchs apart and not more than 12 inches, with the middle being at the middle of the roof. The outside 2 must be at the outer edge, but vague on the location. I have the manual, so I didn't try to find it online.
Be careful saying a 2500 does not require it. Virginia is enforcing the rule about having the roof lights if you pull a trailer over 80 inches wide. You don't need the rear bar unless the truck is over 80 inches wide (mirrors do not count), but dually fenders will put you out of compliance. I've been stopped and was given a warning by Virginia DOT at a road side inspection. Since then I have made a light bar to mount on the roof that meets the requirement.