Although I am 50 years old, I do distinctly remember what life was like attending college from the ages of 17 to 21. Your "children" are young adults. Also, if grandpa is single and still doing the tango, he may wish to have his privacy.
So, the most economical thing to do is go for three inside guarantees. Book three adjacent staterooms of whatever configuration if you feel you need to have the staterooms near each other.