simple: $\begin{array}{ccc}{a}_{11}& a_{12}& a_{13}\\ b_{21}& b_{22}& b_{23}\\ c_{31}& c_{32}& c_{33}\end{array}$

left, default (center) and right column alignment: $\begin{array}{lll}{a}_1& a_{12}& a_{13}\\ b_{21}& b_2& b_{23}\\ c_{31}& c_{32}& c_3\end{array}\text{,}\begin{array}{ccc}{a}_1& a_{12}& a_{13}\\ b_{21}& b_2& b_{23}\\ c_{31}& c_{32}& c_3\end{array}\text{,}\begin{array}{rrr}{a}_1& a_{12}& a_{13}\\ b_{21}& b_2& b_{23}\\ c_{31}& c_{32}& c_3\end{array}$