Tri-Rail for standard NERF rail. I make this for Roughcut, but it can be used for a lot of Nerf guns and rifles.
I used three 14 Dart holder (http://www.thingiverse.com/thing:1386042), with different color (yes, i'm Italian ;-) ) for hold a lor of dart, but it can be used also with all of other Nerf accessories (see photos for some examples).
(The thing in the photos is not the final version, that you can instead find in STL file)