1) Find the eigenvalues of the matrix. The fact that this matrix is not invertible tells you that 0 is one eigenvalue and that helps find the other 2.

2) In fact, 0 is a double eigenvalue, but you can still find three mutually orthogonal eigenvectors of length 1. Find them.

3) The matrix having those eigenvectors as columns will "orthogonally diagonalize" this matrix.