check filename args. otherwise if one of the 3 filenames is bad
        it's hard to tell which one

def _check_input_files(nspc, parser):
        """check filename args. otherwise if one of the 3 filenames is bad
        it's hard to tell which one"""
        if not len(nspc.filenames) == 3:
            parser.print_help()
            msg = """
      3 Expected files; Expected content: study population association",
      {} Actual   files: {}""".format(len(nspc.filenames), ' '.join(nspc.filenames))
            raise Exception(msg)
        for fin in nspc.filenames:
            if not os.path.exists(fin):
                return "*{}* does not exist".format(fin)

        return False