I'm pretty sure I started seeing the trio at Rivbea in '74, and through '75, numerous times. On weekends the trio, or an augmented group with Warren Smith or Joe Daley or both or others, or Winds of Manhattan would often do one set, and a guest artist would do the other. I remember the Braxton Quartet with Wheeler (the most crowded I ever saw the place), and a Frank Lowe set where a new guy in town, Black Arthur (Blythe) played alto. I'm pretty sure I was no longer a regular there by the time of the Wildflowers sessions.
This confirms a June 1974 date:
http://www.bb10k.com/RIVERS.disc.html#74.06.09
And how about this date from 12/72: those 3 plus Paul Bley!
http://www.bb10k.com/RIVERS.disc.html#72.12.00-1
A little research also shows that the Braxton quartet gig was probably June 8 or 9, 1975. Don't know if Holland and Altschul played with Sam as well as Braxton that night.