Triple Captive Customizeable Cable Holder

Derived from bitswype's very cool design.

Single holder has been tripled (and UNIONed). Knobs have been scaled down so that they don't interfere with each other.

Should be easy to extend my hack to create racks of holders of arbitrary number.

Customization options from original file have been preserved (inside SCAD code). Unclear if that means it'll still work with Customizr, however.