Using list comprehension, tuple unpacking:
>>> a = [('x', 'a'), ('y', 'b'), ('z', 'c')]
>>> b = [1, 2 ,3]
>>> [(x,y,z) for (x,y), z in zip(a, b)]
[('x', 'a', 1), ('y', 'b', 2), ('z', 'c', 3)]
>>> a = [('x', 'a'), ('y', 'b'), ('z', 'c')]
>>> b = [1, 2 ,3]
>>> [x + (y,) for x, y in zip(a, b)]
[('x', 'a', 1), ('y', 'b', 2), ('z', 'c', 3)]