g = generator1, generator2, generator3
res = [e for e in itertools.chain(*itertools.zip_longest(*g)) if e is not None
    ]
print(res)
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]
Traversing a sequence of generators