From the wing patterns alone here, one could (with careful scrutiny and ample patience & perseverance) identify these as Trupanea using the complete collection of wing pattern images in Foote, Blanc, & Norrbom(1)(1993)...and the outer two as T. nigricornis, and in inner one as T. bisetosa.

These were among a large group of flies that were found within contiguous chambers separated by thin plugs of chewed plant fiber in a hollowed-out stem (of Chrysothamnus = Ericameria?) that appeared to be prey provisioned by wasps (Ectemnius or Crossocerus?) for their larvae. For more details, see comments under the 1st thumbnail of those shown below, which are all from the same stem: