Did it stop working suddenly, or are you pairing it for the first time?
If the GoPro had heavy impact there could be internal damage, but I would try this first:
Update your Hero3 to the lastest firmware:
http://gopro.com/update/HERO3
Next follow these instructions to pair your GoPro with your device: https://gopro.com/support/articles/gopro-app-shows-no-camera-connected-message
Section 1:
Wi-Fi Network Can't be Found in Phone/Tablet
If the camera's Wi-Fi network can't be found in the phone/tablet's wireless settings menu, follow these steps:
Turn off/on the phone/tablet's Wi-Fi.
Power off/on the camera.
Make sure the camera is in GoPro App or Phone/Tablet or mode and that you see a Wi-Fi icon in the bottom-right corner of the camera's screen.
Try to connect to the Wi-Fi network with the phone/tablet.
If the network is not present in the phone/tablet's list, try another phone/table if possible.
Redo the software update.
Section 2:
GoPro App Shows CONNECT YOUR CAMERA Message
If the GoPro App shows "CONNECT YOUR CAMERA", this means that your phone or tablet is not connected to the camera's network. Follow these instructions to resolve the issue.