The set of all values of $\lambda$ for which the system of linear equations
$$x - 2 y - 2 z = \lambda x$$
$$x + 2 y + z = \lambda y$$
$$- x - y = \lambda z$$
has a non-trivial solution:\\
(1) is an empty set\\
(2) is a singleton\\
(3) contains two elements\\
(4) contains more than two elements