The OPN3 soundchip is mostly a light version of the OPNA soundchip with some functionalities removed: no external ADPCM, no Composite Sine Mode and no GPIO.
FM channel 3 is special as it can be split into 2x 2-op chans or 4x 1-op chans.
Relevant datasheets: [[Media:YMF288.pdf|OPN3 (ymf288) (Japanese)]] and [[Media:YM2608J Translated.pdf|OPNA (ym2608) (English)]]