If it's a distributor 4-stroke engine, you will just use any light that says 4-6-8 on it, set it to 6, and dial it to half the target RPM. So if you want 8000, set to 4000.
If it has a "dead spark"; a distributor that runs at crank RPM, just set the shift point normal for the 6-cyl.
If it is "one coil per cyl", you might need a newer light that has a One Cyl and Two Cylinder setting. Hook to one coil 12v lead. If it has one spark per two revs (normal) set to 1 cylinder. If it has a dead spark, set to 2 cylinder.
For two stroke triples, there are both a distributor model and a 3 coil model. You will use 2 cylinder setting for the 3-coil version, and you will use the 6 cyl setting for the distributor model.
Clear as mud?