Microwave does not cook more than 3 seconds.

You may be having a problem with the timer. Since it is telling you "the food is ready" it is not just shutting down with an error or from execessive current draw.

I am guessing that it does start to heat, just stops after 3 seconds. If that is true the magnatron is probably OK and the power supply is also functioning properly. That is good news.

The bad news is the timer is controled by a microprocessor, and the problems you may have with that are not anything easy to troubleshoot by the user. It will require a trained technician to determain the exact problem. it may need a new microprocessor, or more likely , it has another component in the microprocessor circuit that is faulty.

I am sorry to say, this will require service from a professional.

Although this may not be the answer you had hoped to hear, it is the most likely problem, a precise answer to your question in a forum like this is very unlikely to get, considering the symptoms your microwave is having.

If you found this information helpful a good rating is always appreciated. Hopefully the repair will not cost more than the microwave it worth, but considering the prices of new microwaves these days, that is also unlikely.

If I can be of any more help feel free to ask.