Enter your 2 addresses. Once the 2nd address is entered a third option shows up under the 2nd address. There is a + sign in a circle & beside it reads “Add destination”. Click on the “Add Destination” & another blank line appears where you can add the 3rd address. There are up/down arrows so you can move them into the preferred order & “X” so you can delete. Then proceed as normal.

On my PC I right click on the map, raising a pop-up menu which includes “Add destination”.