In some rare cases you may notice that videos recorded with your HERO3 camera may appear as a mirror image of how they should appear. These images may be flipped horizontally along the vertical axis.
This happens significantly less often with the latest update, so make sure you have updated to the latest software version.
If the issue persists you can use GoPro Studio to fix it. In Step 2 of the editing process there is an option to "Flip Horizontal". Select this option to flip the video so that it looks correct