Question 1
Bob can decode this message by multiplying the vector he receives by the inverse of the encryption matrix. He'll need to do this by applying the inverse to three of message entries at a time. Then he simply needs to substitute the numbers for their corresponding letter in the alphabet.
Question 2
No, eve needs to find the inverse of the secret message to decrypt any messages. However, she can easily find the inverse with what she knows.
Question 3
Using the the input and output matrices that she knows, Eve can determine that the secret matrix is $ \begin{matrix} - \frac{2}{3} & 0 & \frac{2}{3} \\ 0 & 1 & 0 \\ 4 &0 & -1 \end{matrix} $.
She can then find the inverse of this matrix and use it to decrypt the message.
(2,23,3) decrypts to BWE
