In slichos this morning I noticed a mention of the 8th and 9th of Teves also being fasts. Were these three days ever fasts all at once or were these fasts independently observed and replaced each time by the most recent tragedy until the 10th became the most recent fast for this time?
The Behag lists a number of days where the custom is to fast because of various historical tragedies, including both 8 and 9 Tevet. While the Tur and Shulchan Arukh quote this list in OC 580, the Beit Yosef (16th century) and the Arukh haShulchan (19th century) there note that they've never heard of anyone fasting on the days on that list (and frankly, never have I).
I note many question the list's authority as some of the days it recommends fasting on are Rosh Chodesh (when fasting is generally forbidden).
I can't locate my source, but I remember learning that these three days were all fasts for especially pious people (they would fast three days in a row and eat at night.) Since we won't demand that of most people, we fast on the last day of the three to commemorate all three tragedies. So special people did, and perhaps still do, fast on all three days, but that has never been the norm for all Jews.