A diff drop is a good plan on a 3 inch lift.
It restores the CV's / shafts to a better, less sharp ..more OEM angle..
....and will help with CV boot longevity.
You will likely find that your CV boots tear pretty soon after the lift
as they have "assumed' a given angle, and the new angle will stress them.
Doesnt always happen..but often does.
I dont know what the Calmini kit costs these days...
...but a set of springs is under $200 shipped.
If its the full Calminin kit with shocks, its pretty expensive ( $700 ish?)
The shocks are good IMO..I still use them...they can be purchases alone.
Your original x-member is fine.
The Calmini brackets/x-member/extensions etc are good.
.......but really not required.
Jo