3 extruders, how to assign sequence and model part to certrain extruder

[reinier 0]

I'm part of a project that's trying to get an UMO+ to print objects with bigger dimension (20cm×75cm×300cm) by using an external frame that adds a bigger Z-axis and an extra X-axis. I wont go much in dept on this but for this reason and the sake of speed/efficiency I was doing some tests with an unmodified UMO+ and Cura 14.09.

I've got a modelfile containing a simple rectangle split into 3 part (left, middle, right).

In Cura I changed my machine settings to 3 extruders, loaded the 3 stl parts and rightclick selected Dual Extrusion Merge.

Im trying to get it print the left side first, then move to the middle, then the right part and then move up a layerheight, print right layer, print middle layer and the print the left side, etc.

Unfortunately the printer keeps starting with the middle part, then moving to the right one and then to the left, right, middle, right, etc

I've tried importing the STLs into Cura in a different order, which successfully changes the color sequence, but not the actual printing order (still starts with middle)

I've had succes with a box that was made up of 2 parts. There the printer printed left, right, (up) right, left, (up) left, right,(up) right, etc

Any suggestions are welcome and appreciated!