Or search: riant, elated, joyful, joyous, halcyon, cheerful, euphoric, felicity, laughing, contented, felicitous, prosperous
2 Syllables: SU
actress, adage, aptness, Atlas, axis, badness, baptised, Baptist, blackest, blackness, cabbage, cactus, captious, factious, flatness, gaseous, gladness, gratis, habeas, hapless, lapis, lattice, madness, Madras, Magnus, mattress, package, Sabbath, saddest, sadness, slackness, status, taxes, vastness
3 Syllables: SUU
happiness
3 Syllables: USU
compactness, exactness
4 Syllables: UUSU
apparatus, instantaneous

Found 39 words across 4 groupings. Skipped 2 excess words of a similar stem.