How many hours will it take to slow cook 3 pork butts at 8 pounds each on a gas grill?

If cooking all three at once on the same standard run-of-the-mill $100-150 grill/BBQ that most people use on their patio or backyard deck, you'd be better off not using a set period of time as the definitive metric to protect against possible trichinosis. I

Instead, buy an inexpensive meet thermometer designed for use on an open flame grill and cook over low heat/flame -- flipping them occasionally depending on the distance between the meat and the flames -- cook until the most central part of each individual piece of meat reaches an internal temperature of 180-190°F to make sure you don't serve your guests a side of trichinosis with the meaty goodness.

Happy grilling!