If memory serves, Bob Richardson wrote in the Narrow Gauge News that both 315 and 319 were to be scrapped when their flue time was up (this was in about 1949). Needless to say, 315 found a savior, and well, you see what happened with 319. Someone with the books in front of them could give a better account than me, and hopefully they jump in here.
Not sure on 345, but it was probably a similar story. All of these engines pretty much ended as the Durango switcher, as there were not many other roles for the C18s and C19s at that point, with the exception of the Ouray branch and possible lease to the RGS.