It sounds like what you want is a type of trial version for your application. The simplest way to do this would be to write a key to the registry and then add or subtract one from your initial written value for each time the application is run. When you hit zero (if you are counting down) or 3 (if you are counting up), you would stop the application from running. The downside is that the user could change this registry value if they figured out where it was.
For a more complex example, here is a CP article that might help:
Implementing a Rudimentary Count Based Trial Version Plugin for Windows Applications.[
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If you want even more secure, you probably should look into purchasing third-party software for creating trial versions.
Update
Here is a link to a VB.NET solution for tracking the number of days. You could use these concepts and just modify the implementation a bit to get number of uses instead of number of days:
http://www.vbforums.com/showthread.php?621068-Trial-Period-code-VB.NET[
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