ss = [
        ("a1", "b1", "c"),
        ("a1", "b1", "d"),
        ("a1", "b6", "v"),
        ("a2", "b2", "e"),
        ("a2", "b3", "f")
    ]

result = {}
for (a, b, c) in ss:
    if a not in result:
        result[a] = {b: [c]}
    elif b not in result[a]:
        result[a][b] = [c]
    elif c not in result[a][b]:
        result[a][b].append(c)

print(result)