'Can Bob decrypt the message?'
Yes he can. Alice gives Bob the secret Matrix that she is going to be using. So all Bob has to do is find the inverse of the secret matrix after receiving the encrypted message and multiply this encrypted message with the inverse of the secret message to get the decrypted message.
'Can Eve decrypt the message?'
Eve can decrypt the message as long as she knows the encrypted and decrypted message. Now we know that she knows the above data. So all she has to do is find the inverse of the secret matrix. She cannot do it without finding the inverse of the matrix.
'What is the decrypted message corresponding to (2,23,3)?'
In the previous section the inverse of the matrix has been found. Now (2,23,3) multiplied by the inverse of the matrix gives (2,23,5).
This corresponds to the letters B,W and E.
