I suggest that you create an image file in a separate application to be the frame. So like, three video tracks, in which the foreground one is a .PNG

In your case I might use Photoshop (or freeware like Photopea) to take a screen-sized black background and cut out transparencies that the videos will show through. In your scenario that might leave, say, just a 10-pixel wide vertical strip in the middle. (Or might mean that plus some border around the whole thing.)

Then in Resolve, with that frame in the foreground, you can just Transform and/or Crop the videos to fit behind the frame and they’ll look perfect every time.

Hope that made sense! It’s been my method for virtual choirs, etc.