Before buying a new test or subscribing to other tests, I would suggest you take one more mock and see how you score on that. Again, you need to take the MOCK in the same condition that you are forced to take in the actual GMAT.
Buying a new test would be waste of money as well without doing the following.
I would suggest you to analyze what went wrong and what were your mistakes on the finished mocks. And improve on those mistakes before taking the 4th one. By this means, there are chances of score getting improved.
For the resetting part, i wouldn't suggest that though, since you have already seen those questions and there are chances of these questions reappearing. So it is better to analyze those mistakes you made on the previous test and try not to repeat them on the remaining one.
As there is no point in buying the new tests, without analyzing and correcting your mistakes on the existing one and expecting a higher score.
This is just a suggestion. But if you wish and if you don't feel right, you can very well buy a new one

Hope it helps. And all the best.