Or search: tested, trusty, reliable
3 Syllables: SUU
enviable, genital, plentiful, sensible, sensual, sentinel, temporal, tenable, tentacles, ventricle
4 Syllables: USUU
amenable, commendable, congenital, contemptible, conventional, defensible, dimensional, insensible, intentional, lamentable, ostensible, polemical, presentable, unenviable, unquenchable, untenable
5 Syllables: UUSUU
comprehensible, indefensible, indispensable, reprehensible, unconventional, unintentional

Found 32 words across 3 groupings. Skipped 1 excess word of a similar stem.