Templates are a great feature when you need to to repeat a layout with different images.
You can, of course, make a template layout of multiple pages with different sizes on each page.
Also if you want several prints of different images with one particular template layout you can do this by first setting the thumbnails of the images and then recalling the template layout.
In your example for say, for 3 different selected images, the 3 pages would be generated as soon as you recall the layout.
Have a play
Terry