Anders.. This is a great question, and begins to separate the beginners from advanced users. The ability to plan, track actuals, and assess future requirements is a key benefit of using Microsoft Project, but requires some expertise and a bit of care. With that said, let me provide a suggestion to help you with the scenario you mention here.
–Planning the resource: Setup the resource for the three months, but then contour the requirement using a usage view. Setup a resource or task usage view, monthly timephase columns (or to whatever detail you want to provide, and override the flat hourly requirements with your specific requirements. **make sure you baseline your project once this is complete**
–Tracking: Enter actual hours per period and move remaining work to future periods. This is a common activity but feel free to ask if you are unfamiliar with what I’m saying.
–Future requirements: Entering actual hours and moving remaining hours will likely move remaining allocation further out or closer in depending on hour variations. You can go back to the usage view or one of many options to adjust future requirements to meet current estimated remaining effort/duration.
You can now compare actuals and future estimates to the baseline to see how well your resources are meeting expectations. One other, simpler option, is to create two tasks…one full time developing the deliverable and the second for support. Hope this helps…