Or search: organism, throwback
3 Syllables: SUU
actresses, aggregates, analyst, apposite, Arabic, asterisk, candidate, catalyst, mattresses, naturalists
4 Syllables: USUU
anatomist, Annapolis
5 Syllables: UUSUU
Indianapolis, Minneapolis

Found 14 words across 3 groupings. Skipped 2 excess words of a similar stem.