3 spouses, all born in same month & year

My father had three wives, all born in the same month of the same year (they do NOT all have the same sun-sign though).
All 4 of these people were born within a 40km radius of each other.
(I don't have, and am unable to get, birth times for any of them, and for one of the wives, I only know the month & year of birth, not the day).

Has anyone ever looked into the astrological significance of people who repeatedly have relationships with those of a similar birth date?

Hi. The locations are irrelevant without birth times. If you look at the charts of each of these partners - perhaps using the noon chart for the day of birth in the cases where the time is unknown - you can see which planets didn't move much during the time period within which these births occurred, and then see if they make strong aspects to anything in your father's chart. Probably Saturn, Uranus, Neptune and Pluto will be the only ones worth looking at.

Best wishes

Miquar