Python Remove last 3 characters of a string

I’m trying to remove the last 3 characters from a string in python, I don’t know what these characters are so I can’t use rstrip, I also need to remove any white space and convert to upper-case

an example would be:

foo = "Bs12 3ab"
foo.replace(" ", "").rstrip(foo[-3:]).upper()

This works and gives me BS12 which is what I want, however if the last 4th & 3rd characters are the same I loose both eg if foo = "BS11 1AA" I just get 'BS'

examples of foo could be:

BS1 1AB
bs11ab
BS111ab

The string could be 6 or 7 characters and I need to drop the last 3 (assuming no white space)

Any tips?