To my knowledge (and I'm quiet new to LR, too) it is not possible.
Either you choose Single Image / Contact Sheet and print all selected images automatically, but you are restriced to 1, 2, or 4 pictures per page and can't do 3, or you create a Custom Package where you can greate a 3 image layout, but you have to insert the images manually.
If it is just for cutout you could do the following workaround:
Choose Single Image, 1 row, 2 columns, and set the bottom margin so that you have both images in the top area of the page.
Select 2/3rds of your images and print them, the bottom area of each page is blank.
Now create a new layout with just one image, now horizontal in the blank area of the page (either in the bottom area or in the top if you turn your sheets around and print the rest 1/3rd of the images in the blank space.
Now it's your turn to decide what you prefer, handling each sheet twice or drag your pictures manually to a custom package (honestly, I would make a custom package).
Hope it helps