How to force a window to open as a tab?

Using Xcode 11 for a macOS app and storyboards I created an app with three window controllers/view controllers. When first testing the second and third window would open in Tab of the first window, but along the way I changed something on one of the secondary windows and now that window opens in its own window. (Did not notice exactly when a change or changes caused this to happen)
I have a buttons in the main window tool bar that when clicked it provides a segue to the second and third window.

Also, I have tabbing mode on all three set to automatic and I have tried preferred with all windows but the second window still opens in its own window.

I realize this does not contain much detail, but I was hoping someone had seen this before and could help.

How can I get this window to open as a tab as it originally did?
I have looked at tab view to maybe redo them but this is not exactly what I was looking to do.

Thanks
How to force a window to open as a tab?
 
 
Q