I've got it!!!
Make them like the chin gun on the B17G. By that I mean put a gun in. Then slave 2 more guns on each side (the cheek guns) so that, IF they can, they point in the direction of the gunner.
THEN, just remove the gun. Now you have the 2 cheek guns, firing IF they can, but only 1 position that needs to aim.
Just a thought. They're so narrowly spaced that convergence really isn't an issue, especially considering the guns are pretty close to the gunner's position.