Here is a simple approach using the built-in Python .isdigit(), .isalpha() and .zfill() methods. This approach uses the Python parser.
Pre-logic code blockScript Code:
def changeAddress(y):
integer = ''.join(x for x in y if x.isdigit())
string = ''.join(x for x in y if x.isalpha())
final = integer.zfill(3)
return final + string
changeAddress(!YourField!)
