Interesting how three separate population groups didn’t inter-mix much. Perhaps they had opposing cultures also? My haplogroup r1a1 came out of Eastern Europe through the Baltics to settle southern Sweden and Norway. I know they were more recent and brought the Germanic culture with them.
The barriers were probably cultural -- common distant ancestors, but in the interim isolated in some other part of Eurasia -- and language/dialect (ditto), and DNA (ditto). However, if there were intermarriages, the couple would live in one or the other place, and that would be the expressed culture.