This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
仨 is a rare character composed of 亻 (person) and 三 (three). Its historical origin and original meaning are uncertain.
A person 亻 beside the number three 三. Without a clear meaning, you might imagine three people standing together, but the true meaning is uncertain.
For サ, picture three people saying 'sa' in unison: the sound 'sa' cues サ while the three 三 people 亻 chant together.
—