What about
Anna,
Sylvia and
Sally (full name)? Same two S- names, but
Sylvie is more obviously a nn than
Sally, which is totally independent of
Sarah, I'd guess 98% of the time.
I'd be cautious about using two matching initials and leaving
Anna as the odd triplet out. Maybe go 3 x S and have
Suzanne,
Sylvia and
Sally?