You can cook sweet potatoes in a 6 or 8 quart Instant Pot. Since big whole potatoes won't properly fit in smaller units, I do not recommend 3 quart Instant Pot for cooking sweet potatoes. Adjust amount of water depending upon the size of the unit. Add 1 cup to a 6 quart Instant Pot and add 1.5 cups to an 8 quart Instant Pot.