If I understand your post correctly , your car failed it's NCT 3 times. First 2 for a brake issue and now for corrosion. If we assume the corrosion was in place for the first 2 times then it should have been spotted but it wasn't. Potentially it worsened between the first and 3rd test. Why are you looking to be compensated if your car is not roadworthy? That's not the NCT's fault. Even if they had spotted the corrosion the first or 2nd time, your car would still have failed.
You could complain over the quality of the testing but the reality is, your car would still have failed.