Add the potatoes, peas, and ¼ tsp salt to a large saucepan and cover with water. Place over medium-high heat and bring to a boil. Reduce heat to medium and simmer until potatoes are tender, about 15 minutes.
Drain the potatoes and peas reserving about 1 cup of the cooking water.
Put the drained potatoes and peas in a large bowl and mash well with a potato masher.
Stir in the corn then a little of the reserved water until you reach the desired consistency.
Stir in the butter and season with salt and pepper to taste.
