Try this,
x in mylist is better and more readable than x in mylist[:] and your len(x) should be equal to 3.
>>> mylist = [[1,2,3],[4,5,6,7],[8,9,10]]
>>> for x in mylist:
... if len(x)==3:
... print x
...
[1, 2, 3]
[8, 9, 10]
or if you need more pythonic use list-comprehensions
>>> [x for x in mylist if len(x)==3]
[[1, 2, 3], [8, 9, 10]]
>>>