Find the dimension of the space
\[
U=\operatorname{span}\left\{\left[\begin{array}{l}
0 \\
3 \\
1
\end{array}\right],\left[\begin{array}{l}
1 \\
1 \\
1
\end{array}\right],\left[\begin{array}{c}
2 \\
-1 \\
1
\end{array}\right],\left[\begin{array}{l}
3 \\
0 \\
1
\end{array}\right]\right\} .
\]