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2 Syllables: SU
brightening, cycling, firing, frightening, heightening, hireling, hiring, idling, ironing, ripening, stifling, tightening, tiring, wiring
3 Syllables: SUU
hyacinth, lightening, quieting, rioting, widening
3 Syllables: USU
acquiring, admiring, aspiring, conspiring, desiring, enquiring, entitling, expiring, inquiring, inspiring, perspiring, requiring, retiring
4 Syllables: USUU
disquieting, enlightening, enlivening

Found 35 words across 4 groupings. Skipped 1 excess word of a similar stem.