TFTT; I just emailed to you a photo of a triple tunnel from an unspecified geo location.
If others would search for "Railways of Germany English triple track tunnel" and slide down the page you will see a fantastic prototypical triple tunnel. Also some nice three track model examples. I have the photo but can not post it per copyright rules.
Due to the oblong / oval nature of a three track tunnel it would rise well above the typical 6" clearance used by many.