Bob can decode the message
All Bob has to do is invert the matrix and multiply the code with this inverted matrix 3 numbers at a time. Finally he can obtain the message by converting each number to it's corresponding alphabet character.
Eve can decode the message without the secret matrix's inverse
Since she knows both the original message and the decrypted version, she doesn't really need the secret matrix to find the original does she? She already has it! All she has to do is think about it, perhaps meditate on the matter, and realize that each number corresponds directly to a letter in the alphabet.
