It seemed like the the trio (seasons 1-3) were written as as having one combined personality, rather than having three distinct personalities.
Yes, there were some differences, but they were mostly blended into each other.
When Shelly Hack was cast in season 4 as the east cost, more refined one, it didn't work because the writers didn't develop
the contrasts, which could have given the series some interesting dynamics.
Interesting that at the beginning, Kelly was supposedly meant to be more street smart, but it was not Jaclyn Smith's
strength. Kelly was written as more refined which made Tiffany redundant. Maybe a more street wise character was the way to proceed,
and Julie was that to an extent in season 5, but the show had run its course by then. Whatever was planned it just did not work