Anyone have any instructions on how to modify a 3 button genesis/megadrive controller to work on the marty?
The Marty uses the same interface as other Japanese PC's. If you use the same wiring method for using Mega Drive controllers on an X68000 you should be good.
The simple adapter route for getting Genesis pads working on the FM Towns doesn't seem to work. I modified a clone Genesis pad (the everpresent Hyperkin GN6) for X68000 use and it won't work on the FM Towns. I tried a normal 6-button Genesis pad, a GN6 clone and a third party autofire pad (SG Propad 6) through an adapter and none of them will work. All of this works just fine on X68000, but on the FM Towns I don't get any response at all.
According to this post (https://yesterware.blogspot.com/2020/12/fm-towns-and-capcom-power-stick-fighter.html), the only adapter that actually works is the Capcom CPA 001, the expensive/rare official adapter meant for use with the CPS Fighter fightstick.
There's another possibility - Fujitsu made an official 6-button pad near the end of the FM Towns' lifespan. It's got the model numbers FMT-PAD601 and FMT-PAD111. It's quite rare and expensive, but games do have explicit support for it. It's totally different internally from the Sega 6-button pads but the full schematic (http://www.awa.or.jp/home/shimojo/tow6kai.htm) is known. The next step would be to take a cheap Genesis pad and rewire it or create a whole new PCB for it. The Hyperkin pad is an excellent candidate for this, though without spinning up a replacement PCB it's going to involve a ton of cutting and jumpering.