Or search: option, default, obverse, deciding, druthers, disjunctive, impossibility, unconventional
3 Syllables: SUU
burgesses, furnaces, purchases, purposes, surfaces, surpluses
4 Syllables: USUU
affirmative, alternative, conservative, disturbances, divergences, interpretive, observances, preservative, superlative

Found 15 words across 2 groupings. Skipped 1 excess word of a similar stem.