def f(*args):
    result = []
    for x in args[0]: 
        if x in result: 
            continue 
        for word in args[1:]: 
            if x not in word: 
                break 
        else: 
            result.append(x) 
    return ''.join(result)
s1, s2, s3 = "message", "massage", "mask"
print(f(s1, s2, s3))