You really do not need it:

a.extend(b).extend(c)

Or sorted:

a.extend(b).extend(c).sort()

Or unique:

list(set(a.extend(b).extend(c)))

For this useful feature with chain

>>> from itertools import chain
>>> a = [0, 1, 2]
>>> b = [2, 3, 4]
>>> c = [4, 5, 6]
>>> list(itertools.chain(a, b, c))
[0, 1, 2, 2, 3, 4, 4, 5, 6]

or from a sequence of sequences:

>>> list(itertools.chain.from_iterable([a, b, c]))
[0, 1, 2, 2, 3, 4, 4, 5, 6]

Note that chain() does not remove duplicates and returns an iterable, not a list.

I'm curious, why write in english? I think it's the first post I see in that language (in that site of course). That's all.