You don't "add a third state".
In the personal info section, of TurboTax (TT), you indicate your current address, in SD. You then indicate that you previously lived in MD. There is no where to tell TT that you lived in MI. But, you do tell TT that you made money in MI (if you had income for that period). That will generate MD & MI software. Any additional info needed will be covered in the state interviews (you can indicate that you were a part year MI resident).
