Q: Magic mouse does not work. Changed batteries three times. Steady green light for a few seconds and then flashing green light.

Did you pair it in BlueTooth preferences?

Bluetooth- How to set up your Apple Wireless Mouse or Magic Trackpad

I had the same problem. I restarted my computer (by holding the power button in the back to turn it off and then pushing it again to turn it back on). Now it's working. I think it lost the connection and when the computer restarted, it found the connection with the mouse.