If there's a third target and you still haven't fired all ten shots, keep shooting. As long as the target is in line with the intended target, you keep firing until you've used up ten shots. Mortars fire high angle, and their shells land on top of objects. They cannot fire directly, but must lob shells in an arc. A mortar has the same range as a cannon, but it cannot fire at targets less than R1 from it.
Mortars can fire over walls, however.
They cannot fire grapeshot. The third nearest the shooter is Close range, followed by Medium and ended by Long range. When you fire a gun, its range affects how well you hit target. When you shoot a target, roll two dice. At close range, you need 6 or more to hit.