Scrub potatoes and slice into 1/4 inch slices. Potatoes may be peeled if you prefer. Cut peppers and onions into 1/2 inch dice. Combine potatoes, onions and peppers on a double thickness of foil. Add salt and pepper to taste. Dot with butter. Fold the foil sealing the edges to make a leak proof packet.
Preheat grill to medium heat. Place packet on hot grill rack. Cook turning occasionally 20 - 30 minutes until potatoes are tender. (you should be able to pierce with a fork). Potatoes may be moved to a top rack while you cook your burgers, chicken, etc.
Remove packet from grill. Use caution! Contents are hot. I usually use a baking sheet to transport the potatoes. You can serve the potatoes right from the foil packet, or put in a serving dish. I opt for the foil. Less clean-up.