Place the potatoes in a large saucepan and add water to cover. Stir in the salt and place over medium flame.
Bring to a boil, then reduce heat and simmer add in the peas and cook until potatoes are tender. Drain, reserving the liquid.
Simmer the corn with a little salted water in a separate saucepan for a few minutes while the potatoes and peas are cooking.
Mash the potatoes and peas together with a potato masher to form a pale green puree.
Stir in the corn with a little of the reserved liquid and season to taste with salt and pepper.
Add a pat of butter to taste.