I understand that your motherboard stop responding after working normally for 3 years.
In regards to the warranty, Intel(R) offers a 3 year warranty on most of the products but usually the products last longer as it depend on the usage that the system has, temperatures and devices connected. All of these will affect the lifespan of the products.
Now regarding the problem that you are experiencing, there are many troubleshooting steps that can be done to determine if the motherboard is indeed defective.
I recommend testing the system out of the chassis on top of a non-conductive surface; this could be a wooden table or a newspaper. Also while you are taking the motherboard out of the chassis, you can clear the cmos by removing the power cables from the motherboard ( 2x12 and 2x2) and the cmos battery for an hour. Once you have the motherboard out of the chassis and cleared the cmos, we need to test the motherboard on a basic configuration, this means have the system as basic as we can. Have only the power supply and the processor with its Heat Sink. We won't need memory or any other device as we are seeking for 3 long beep tones. If you receive the beep tones, then turn off the computer and test with one memory stick and the monitor connected.
If after these troubleshooting steps you don't een have fan spin you can try replacing parts like the power supply or motherboard.