Is there any way that a 20b can be machined to accept 3 spark plugs per chamber like the RB26 four rotor has?

 

I doubt it. And besides, why bother?

I have given a 20B ignition system much thought, and I've figured out a way to have dual Leading sparks for all 3 plugs, and a single spark for all Trailing plugs so it will run with as much ignition power (per rotor) as any normal 2nd gen RX-7, or any '81-'85 distributor running in direct fire.

Oh, and I stayed away from the whole MSD direct fire ignition system because it's faulty.