Or search: smuggling
2 Syllables: SU
bungling, crumbling, fumbling, grumbling, humbling, mumbling, rumbling, stumbling
3 Syllables: SUU
blundering, comforting, functioning, lumbering, numbering, plundering, slumbering, summoning, thundering, tumbling, wondering
4 Syllables: USUU
encompassing

Found 20 words across 3 groupings.