If you count by threes, the ones place is unique until you pass each multiple of 30 - 3, 6, 9, _8, _1, _4, where was I? I hadn’t made it to 30, ends with a 4, must be 24. 27, 30, 33, _6…

Serves as a sort of checksum, as long as you know roughly how many you have and just the last digit.