Wash the potatoes. Put them into a pot and add salt. Pour water until potatoes are covered and bring to a boil. Cover the pot and simmer over medium-high heat until completely cooked through.
Drain water from the potatoes and peel them. Then mash the potatoes with a potato masher. Add the melted butter and milk to the mashed potatoes and mix well.
Whisk together the mashed potatoes, eggs, cheddar cheese, mozzarella cheese, Parmesan cheese, green onions and sour cream. Season with salt and pepper if needed.
Preheat oven to 375 degrees F. Grease a muffin pan really well. Spoon the mixture into the prepared muffin pan.
Bake in oven until the tops are golden brown and crisp around the edges, about 30-35 minutes.
Remove the muffins from the oven and allow them to cool in the pan for 5 minutes. Transfer the muffins to a serving dish, top them with the sliced green onions and serve with sour cream on the side.