There are a few instruments with triple string courses. I believe John Denver had an 18 string guitar with triple string courses.
The Canadian painter Tom Thompson owned a 12 string mandolin with triple string courses. It resides in the Tom Thompson Museum in Owen Sound, Ontario.
My tiple has 2 double string courses and 2 triple string courses and is tuned gG-cCc-eEe-AA.
I have a Filipino banduria with a single bass string, two double courses and three triple courses that I tune D-GG-CC-FFF-AAA-DDD.