It's possible they're all past their shelf life? But unlikely in the case of all 3 RTC's, from 2 different vendors?

What should I test to further evaluate?

ASIDE: the OS (Raspbian 2017-11-29) retains the time on disk/microSD upon next bootup ("time literally stops until then!") [Possibly due to "fake-hwclock" ? Which acts a (bit) like a fake NTP server...] So if it's any consolation at least time does not go backwards (as was typical in older OS's :-)