At the bottom right of the desktop area there is a "Recent Items" area which can be expanded or collapsed. When you move to the next item the current item will be moved to this area. Each recent item has a play overlay in the middle that when pressed will bring that item back to the presenter area at the point it was left. Activating the item from the playlist will always start from the initial slide.
So the flow in your situation would be to show the powerpoint and at slide 3 you bring the song up from the playlist. When you have finished the song you will click play on the PowerPoint thumbnail in the recent items are to continue from slide 3.