as a slight nudge towards the answer, solve the system:
(A - λI)v = 0. in this case, λ = 3, so you must find the null space of the matrix:
$\begin{bmatrix}1&2&3\\-1&-2&-3\\2&4&6 \end{bmatrix}$
the rank of this matrix should be obvious upon inspection, and the rank-nullity theorem then tells you how many basis vectors you should have for the null space.