Or search: gravity, sincerity, staidness, solemness, earnestness, seriousness
3 Syllables: SUU
centuries, century's, densities, density, embassies, emphases, enmities, enmity, entity, Kennedy, remedied, remedies, remedy, sensibly, splendidly, tendencies
4 Syllables: USUU
amenities, contentedly, dependencies, extremities, extremity, identity, indemnity, insensibly, lamentably, nonentity, ostensibly, parentheses, propensities, serenity, solemnity
5 Syllables: UUSUU
indispensably
6 Syllables: UUUSUU
plenipotentiaries

Found 33 words across 4 groupings.