Less of a thing nowadays, but it's still fun to see multiple shapes on a post, too. I don't think there's an alignment in California that currently signs a US highway, an Interstate and a state highway simultaneously, but there are plenty of twosies.
Not concurrencies per se, but the BGS examples of 3 route types on one sign in this state:
50/99/(formerly Business 80) in downtown Sacramento
101 TO 5/10/60 along the Santa Ana Freeway in downtown Los Angeles (a signage change that occurred in the last few years)
Right, but there's no reassurance shields for these; they're just on BGSes, as you say.
50/99/Business 80 trio signposts did exist along W and X Streets in Sacramento for a while, though not on the WX Freeway itself as far as I can remember - I feel like in the time I lived in Sacramento, there weren't any trailblazer shields along the WX at all (but some on the West Sacramento portion of what was Business 80 and is now just solely US 50).
I-5 and I-10 at the East Los Angeles Interchange might be the other example of a concurrency whose entire signage was relegated to BGSes with no trailblazer usage, but that would not be easy to prove unless there were 1960s-1970s Caltrans photos of that southernmost part of the Golden State Freeway. (This can be contrasted with the Eastshore Freeway in Berkeley, which has always had a concurrency of some sort: US 40/Route 17, US 40/Route 17/I-80, Route 17/I-80, and now I-580/I-80 - the latter of which is DEFINITELY signed with trailblazers)
When was the last three-route-type concurrency with reassurance shields in CA - the old 15/18/91/395 setup near Colton (now 215)?