Given vectors \(\mathbf{a} = \mathbf{i} + \mathbf{j} - 2\mathbf{k}\), \(\mathbf{b} = \mathbf{i} + 2\mathbf{j} - 3\mathbf{k}\), \(\mathbf{c} = 2\mathbf{i} - \mathbf{j} + \mathbf{k}\), and \(\mathbf{r}\) such that
\[ \mathbf{r} \cdot \mathbf{a} = 0, \\ \mathbf{r} \cdot \mathbf{c} = 3, \\ [\mathbf{r} \quad \mathbf{a} \quad \mathbf{b}] = 0, \]
Then find \(|\mathbf{r}|\).