Final answer:
The maximum number of codes this security system can generate is 17,576.
Step-by-step explanation:
A three-letter security code can have up to 26 choices for each letter, as there are 26 letters in the English alphabet. Therefore, the maximum number of codes this security system can generate is calculated by multiplying the number of choices for each letter. In this case, it would be 26 x 26 x 26, which equals 17,576.