Repeat Steps 2 and 3 to create Main Windows 2 and 3, respectively. On Main Window 2 make sure you have a button for both Main Window 1 and Main Window 3. Set your window path for the Main Window 1 button to Main Windows/Main Window 1 (refer to step 4). For the Main Window 3 button, set the window path to Main Windows/Main Window 3.
In the following example, Main Window 2 is open. By clicking either button, it will navigate you to the respective window. Click on the Main Window 1 button, and Main Window 2 swaps out and opens Main Window 1. Click on the Main Window 3, and Main Window 2 swaps out and opens with Main Window 3. You'll notice that you always have one main window open.