We know that a diagonal matrix has only a₁₁, a₂₂ and a₃₃ for a 3×3 matrix such that these elements are equal or different and all other entries 0 while scalar matrix has a₁₁ = a₂₂ = a₃₃ = k(say). So, a diagonal matrix which is not scalar must have a₁₁≠a₂₂≠a₃₃ for i≠j

Required matrix =