Or search: perk, apanage, assignment, perquisite
2 Syllables: SU
average, fastenings, happenings
3 Syllables: SUU
absences, gatherings, labyrinth, mattresses, narrative, Nazareth, palaces, pasturage, patronage, vassalage
4 Syllables: USUU
anatomist, collaborative, comparative

Found 16 words across 3 groupings. Skipped 2 excess words of a similar stem.