Use a list comprehension to filter elements, keeping everything not length 3:
x = [el for el in x if len(el) != 3]
Demo:
>>> x = [('x1', 'x2', 'x3'), ('x1', 'x2'), ('x2', 'x3'), ('x3', 'x4')]
>>> [el for el in x if len(el) != 3]
[('x1', 'x2'), ('x2', 'x3'), ('x3', 'x4')]