import re
filenames = ['122 CHC Sep 2017.xlsx', '124 CHC Sep 2017 RFK.xlsx', '124 CHC Sep 2018 Trc.xlsx']
rx = re.compile(r'^\D*(\d+)(?:.*?(\s*[a-zA-Z]+)|.*)\.[a-zA-Z]+$')
print([rx.sub(r'\1\2', x) for x in filenames])