3 6 9 refers to the location of subdials. 3 o clock 6 o clock 9 o clock.
If I type 12-6-9 7750 this is the native configuration of the 7750 with subdials at 12 o clock 6 o clock and 9 o clock. This is the original configuration.
3-6-9 7750 is generally considered more unstable due to the transfer gear required to shift the subdial location and also leads to a sunken date
Automatic refers to automatic movement