Two secondary coils instead of one. Each only has to hold 18 in of spark, so the pair reaches 36 in combined, with the voltage divided across them rather than doubled on one. It uses dual H-bridges of IRFP260 MOSFETs and MUR1560 diodes. The controller is reused from SSTC-2 with a staccato circuit added for power control, and feedback is taken off just one of the two secondaries. The centre point between the coils floats on purpose: “grounding the center of the 2 coils in fact reduces performance.”
Schematics and photos
SSTC-3 · Dual H Bridge 1 
SSTC-3 · Arc 1 
SSTC-3 · Arc 2 
SSTC-3 · Arc 3 
SSTC-3 · Box 2 
SSTC-3 · Box 3 
SSTC-3 · Box 4
Sources
This is the card from the driver tree, written from what the builder published. Send corrections through the forum.