1 Syllable: S
bout, bowed, cloud, clout, couched, cowed, crouched, crowd, doubt, drought, Faust, gout, loud, oust, out, ploughed, plowed, proud, rout, route, scout, shout, shroud, snout, sowed, spout, sprout, stout, tout, trout, vouched, vowed
2 Syllables: US
about, allowed, aloud, avowed, devout, endowed, espoused, Macleod, redoubt, throughout, without
3 Syllables: UUS
disallowed, disavowed, thereabouts

Found 46 words across 3 groupings. Skipped 12 excess words of a similar stem.