One of the three can be found near the farm south of Falador, directly north of Port Sarim in the cow pen, a Churn is nearby. The quickest way to get to there is to cabbage port with explorer's ring 3. There is also one in Lumbridge, by Mill Lane Mill and Fred the Farmer's sheep farm, and another north of Seers Village inside the Sinclair Mansion. In some of the starter quests, milk is required, and so the Milk sellers were added so it would be easier to find milk, other than milk a cow.