There is more than one horizontal 3n layout, depending on whether it starts at the top or bottom and left or right.
If you can visit the G+ Eigenharp community, you will find a discussion on this from a few days ago with examples and links. Technobear posted some Belcanto scripts there (generated from his setup Creator utility) that you can download and execute to try a couple of variations. Basically these modify the course offsets and musical mapping for keygroup 1.
The Belcanto lines are pretty long. I had difficulty getting them to execute correctly in EigenCommander, but eventually succeeded using brpc after troubleshooting in Workbench. I don't know why they would work in one and not the other.
If you have Workbench, you can edit keygroup 1 and copy/paste or type just the values for course offsets and musical mapping into the appropriate slots. Workbench also has a mapping editor that is easier to work with than typing mappings into the form. I found it useful for debugging the original scripts, before I discovered brpc.
Anyway here is Belcanto for a Tau musical mapping - horizontal starting from top left (modified from one of Technobear's posted examples because top left orientation makes more sense to me).
keygroup 1 hey musical mapping to [ [[1,1],[1,1]], [[1,2],[2,1]], [[1,3],[3,1]], [[1,4],[4,1]], [[1,5],[5,1]], [[1,6],[6,1]], [[1,7],[7,1]], [[1,8],[8,1]], [[1,9],[9,1]], [[1,10],10,1]], [[1,11],[11,1]], [[1,12],[12,1]], [[1,13],[13,1]], [[1,14],[14,1]], [[1,15],[15,1]], [[1,16],[16,1]], [[1,17],[1,2]], [[1,18],[2,2]], [[1,19],[3,2]], [[1,20],[4,2]], [[1,21],[5,2]], [[1,22],[6,2]], [[1,23],[7,2]], [[1,24],[8,2]], [[1,25],[9,2]], [[1,26],[10,2]], [[1,27],[11,2]], [[1,28],[12,2]], [[1,29],[13,2]], [[1,30],[14,2]], [[1,31],[15,2]], [[1,32],[16,2]], [[1,33],[1,3]], [[1,34],[2,3]], [[1,35],[3,3]], [[1,36],[4,3]], [[1,37],[5,3]], [[1,38],[6,3]], [[1,39],[7,3]], [[1,40],[8,3]], [[1,41],[9,3]], [[1,42],[10,3]], [[1,43],[11,3]], [[1,44],[12,3]], [[1,45],[13,3]], [[1,46],[14,3]], [[1,47],[15,3]], [[1,48],[16,3]], [[1,49],[17,1]], [[1,50],[18,1]], [[1,53],[1,4]], [[1,54],[2,4]], [[1,55],[3,4]], [[1,56],[4,4]], [[1,57],[5,4]], [[1,58],[6,4]], [[1,59],[7,4]], [[1,60],[8,4]], [[1,61],[9,4]], [[1,62],[10,4]], [[1,63],[11,4]], [[1,64],[12,4]], [[1,65],[13,4]], [[1,66],[14,4]], [[1,67],[15,4]], [[1,68],[16,4]], [[1,69],[17,2]], [[1,70],[18,2]]] set
Here is Belcanto modifying course offsets for Tau 3n layout with horizontal left-to-right musical mapping (this is slightly different from offsets for the right-to left layouts due to the not-exactly-a-rectangle keyboard shape):
keygroup 1 hey course offset to [0.0,3.0,3.0,3.0,3.0,3.0,3.0,3.0,3.0,3.0,3.0,3.0,3.0,3.0,3.0,3.0,5.0,3.0] set
Here is Belcanto for Tau 4n layout adjusted to put the root in long column 3 (necessary to make root scales (i.e. C-Major and C-Minor) nicely playable on Tau)
keygroup 1 hey course offset to [-2,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,6,4] set
There is documentation on Keygroup and Course in the wiki, though not thoroughly up-to-date and no mention that I can find there of the new Belcanto array formats above. There is other Belcanto there, however (I had forgotten) for adjusting course mapping and offsets in smaller steps.
None of the above Belcanto lines seem to execute correctly for me in EigenCommander, but they work fine in a script file or in a Mac command line, for instance (replace curly brackets below with angle brackets:
/usr/pi/release-2.0.74-stable/bin/brpc '{interpreter1}' exec keygroup 1 hey course offset to [-2,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,6,4] set