I can't say for sure what will happen if you retake test 3, but there's a decent chance it will be useful, so it's worth a try. If you get ten questions into the test, and you recognize more than one or two of them, it would probably make sense to stop though. An alternative would be to retake one of the free tests. They draw from a bigger question bank than the paid tests, so you're less likely to see repeat questions, and if you took those a while ago, you might not remember repeats well anyway. The only way to find out is to try, but again, I'd abandon a test if you knew too many questions for the test to be useful.
It's fine to take a test when a couple of questions are familiar - just try to pretend you hadn't seen them before, so let a realistic amount of time elapse, and get the question right if you think you'd get it right if you didn't know the question. If you think you wouldn't ordinarily get the question right, use a random number generator to guess (don't intentionally pick a wrong answer, because then you're penalizing yourself more than you should). You can use the last digit of the minutes on the clock to make a random guess (1 and 6 are A, 2 and 7 are B, etc).