I know what you are referring to in regards of playing back videos filmed at a higher fps then 24-30fps.
That is because the human eye can only see 24-30fps. Which is why TV Shows, and even YouTube videos are compressed down to 24-30fps.
According to this GoPro specification chart you cannot record 720p with 24-30fps on GoPro Hero Black 3:
http://i.imgur.com/YvPLc9i.jpg
The best optionI would recommend for you to film at: 1080p 24fps or even higher fps (60fps if you want to do smooth slow motion effect).
The reason for 1080p is because you can crop, zoom, stabilize footage, and etc... down to 720p.
Example:
If you filmed at 720p resolution, and when you get to editing stage you have a unwanted person or object on the sides or corner of the video. You can't crop them out.
If you filmed at 1080p resolution you would be able to do such thing.
But this all depends on how heavy and committed you are when doing video editing. You can simply just record in 1080p at 24fps and just convert the video down to 720p 24fps.
TL;DR:
GoPro Hero 3 Black cannot film in 720p under 60fps.
Best option is to recording 1080p 24fps or higher and convert the video afterwards.
1) Use the GoPro Studio software to convert video down to 720p 24fps.
2) Use Adobe Media Encoder to convert video down to 720p 24fps.
3) Or any other video editing or converting software to convert video down to 720p 24fps.