Thanks, Melierax and Phalarope!
Yes, Cassiar are Oregon x Slate-colored, but the article by Rick Wright that sfinmt linked to says, 'It is worth remarking that the same scenario probably recurs even now, with occasional pairs of "pure" Oregon and Slate-colored Juncos producing young that potentially replicate the appearance of a true Cassiar Junco (Miller 1941 ). With that possibility in mind, it may be strictly more accurate away from the breeding grounds to speak of "apparent" Cassiar Juncos or "Cassiar-type" Juncos, though it seems likely that most such birds are in fact genuine Cassiar Juncos, members of the stabilized and geographically delimited population designated J. h. cismontanus.' Wright talks about that in more detail in his book on sparrows.
Since everyone is wondering... I joined The Junco Complex group on Facebook and asked about these photos. The two responses I got agreed that the first two birds are Cassiar. One said weird intermediate birds like the third should go on eBird as cismontanus for the time being, but the other suggested that J. h. thurberi, the Oregon subspecies that breeds in most of the California mountains, can have gray flanks. So I'm going to put the third photo onto eBird under "Dark-eyed Junco" with the caption "You make the call."
Also I apologize for misspelling "juncos" in the thread title.