why do my speakers keep blowing?

My speakers keep blowing because ....

If you've aren't just trolling, and have actually had three sets of computer speakers blow, then I would be willing to bet that you either have power issues, or the output of your soundcard has some kind of issue.

My money is on trolling though.

borrow someone's amp or computer to test the speakers.  another possible is that the speaker cables run next to a 110v power cord and there is EM leakage from the 110v to the speaker cables

You might have an output for 16 ohm speakers  and using 8 ohm speakers....and want to play it loud.....so boom.... 

Keep the volume on your receiver at least 5-6dB below its max setting. The volume adjustment on most receivers goes from -80dB (mute) to about +16dB (max). By staying below -6dB below that max setting you can typically avoid over-driving the receiver, which could potentially damage your speakers and receiver.

The device you're connecting those speakers to is the problem. If it's an amp, the speakers may be too weak for it. Make sure the ohms is correct and more importantly, the wattage is equal or a bit more than the amp can provide. If its a computer, go to Speaker Setup to make sure the speakers are setup properly.

you are turning the volume up too high for them to handle. no speakers are designed to handle loud music with the knob at max. maximum volume is reserved for when the music is too quiet for you to hear.

either turn the volume down, or buy higher wattage (or higher quality) speakers.