3 Syllables: SUU
merciful, Percival, permeable, personal, purposeful, surgical, terminal, versatile, vertebral, vertical, virginal, workable, worshipful
4 Syllables: USUU
convertible, discernible, impersonal, liturgical, observable, reversible, transferable, unworkable
5 Syllables: UUSUU
interpersonal, irreversible, metallurgical
6 Syllables: UUUSUU
incontrovertible

Found 25 words across 4 groupings. Skipped 1 excess word of a similar stem.