Long time TTRPG player here (3.5/5/SF1), I do figure I might as well give this a shot.
3d6 - 1 + 6 ⇒ (6, 3, 1) - 1 + 6 = 15
3d6 - 3 + 6 ⇒ (5, 4, 3) - 3 + 6 = 15
3d6 ⇒ (6, 6, 6) = 18
3d6 - 1 + 6 ⇒ (5, 6, 1) - 1 + 6 = 17
3d6 - 1 + 6 ⇒ (1, 5, 5) - 1 + 6 = 16
3d6 - 3 + 6 ⇒ (3, 6, 5) - 3 + 6 = 17
... If there ever was a time to play a class that relies heavily on multiple stats, that time is certainly now. Magus?