instead of having 3 different turrents why not just one with different attachments - machine gun, canon & phaser.
Using your own animation - turrent pops up from polygon centre - and choice of gun apears.
3 seperate gun towers and 1 with all guns and detached guns.
made with google sketch up
Free for anyone to use; no credit needed
(However would love to see it rigged, animated, textured or used in something if so please send me a link to it)