Coding replacement fobs to 5AS Mems 3

Hi, I tried to code 3 fobs to the 5AS. The system successfully coded the first two but would not code the third. What I would like to know is was this because the system can only do two or is my third fob faulty. Thanks.

from memory I think you can code upto four, so either faulty or not compatible. There are different part numbers, like 3TXA, 3TXB, 3TXC, 17TN and not all fobs are compatible with all cars. Also there's 315 and 433 MHz.