How can Bob decrypt the message?
Bob can divide the encrypted message into smaller vectors of 3 elements each. Then, multiply each of these vectors by the inverse of the encryption matrix and reassemble the resulting vectors into the original matrix.
Can Eve decrypt the message without finding the inverse of the secret matrix?
What is the decrypted message corresponding to (2,23,3)?
By applying linearity principles we know that
$ \ [2,23,3] = a[2,0,0] + b[0,1,0] + c[0,0,3] $
which yields
$ \ 2 = 2a, 23 = b, 3=3c $
