es1 giovanni

def es1(s1,s2,s3):
    s4 = ""

    for lettera in s1:
        if lettera in s2 and lettera not in s3 and lettera not in s4:
            s4 += lettera

    return s4


a = "albero"
b = "libero"
c = "roma"

d = es1(a,b,c)
print(d)