Peel potatoes, parsnips and rutabagas. Chop all into a large dice, keeping them similarly sized. Peel and dice the onion. Place potatoes, rutabaga and parsnip into a medium sauce pan, add salt, add water to fill just above the level of the vegetables in the pot. Bring to a boil, reduce heat and simmer until tender, about 15 minutes. Drain, place vegetables back into the pot, add butter and cream cheese, then mash all together with a potato masher.