Got stuck with overambitious master thesis

Your situation is quite common with MS theses (in the US at least). There are several people who are given ideas from their advisors which either turn out to be dead-ends or take up much more than the allotted time. In EECS, the MS thesis defense is more about displaying your ability to conduct research than having significant results. Clearly describe the problem, your methods, and display preliminary results whether they are significant or not.

As an example, a professor in my university is interested in inverse problems in imaging. Every once in a while he takes up a student for an MS thesis for denoising images. Problem is, nothing really beats an algorithm called BM3D in terms of PSNR for denoising images (except in specific circumstances). So he has had a few MS students who developed a new method which didn't work as well as BM3D but nonetheless conducted meaningful research and successfully defended. Everyone on your committee knows that it is impossible to anticipate whether an idea will actually work before doing it.

Switching topics and advisor halfway through might not work (how do you know that a new idea is doable in time before actually doing it?). Usually, this is best reserved for situations where there are inter-personal conflicts. Assuming that your advisor is looking out for your best interests, schedule a meeting and talk to them about your concerns with the amount of time you have left and what aspect of the project you should focus on.

I suggest not worrying too much about it. Research is often never finished - you can reach a nice stopping point, but there's always something further you can try. It's not uncommon for a Masters student to graduate and then the supervisor engage a PhD student to keep developing the work.

If you can't complete the research in the remaining 3 months after three months of preparatory work, what are the odds that you'll be able to jump ship and complete a different project in 3 months?

Your supervisor should be aware of how much he can ask from a Masters student. If the conclusion is you've genuinely tried + learned something, but he gave you too ambitious a project, then you'll probably get the MS degree anyway. If you did better than expected of MS students, you might even get a MS degree with honours. Just relax, do your best, and don't worry too much.

tl;dr: Talk to your advisor about how to proceed

Indeed, as others suggest, yours is not an uncommon situation; and in fact many (most?) Masters' projects are actually 1.5-2 years long anyway. But, be that as it may - don't agonize over this alone: You have a supervisor. Use him/her. It's also their responsibility to make sure that you're on track, and it's also their fault if you've gotten stuck in a dead end. Tenured academics are also evaluated by the performance of their grad students, so there's also a "utilitarian" benefit to him/her helping you.

Your advisor is likely to have reasonable suggestiongs that are more specific to your situation rather than the wild guesses we're making.