How can Bob decrypt the message?
Bob can decrypt the secret message by using the inverse of the secret matrix.
Can Eve decrypt the message without finding the inverse of the secret matrix?
Yes, since the matrix is linear, we can write the matrix in to 3 equations and 3 unknowns to solve the 3 entries
What is the decrypted message corresponding to (2,23,3)?
The decrypted message corresponds to the letters B,W,E