Here is some tips for different them:
1. SAE has a black line in middle of body, which will extend into it's tail fin.
Adult SAE will has a brighter body color compare to flying fox
2. Flying fox ALSO has a black line which extend to it's tail fin. Adult flying fox will has a yellowish body compare to SAE. The problem is, when they are young, they looks the same!

Well I read about some article that teach to diff them. But unfortunate i can't identify my flying fox and SAE
3. chinese algea eater. This fish is similar to flying fox/SAE. the diff is it has a black line on it's body, which WILL NOT extend into tail fins. They are much more active than other 2 brothers
Hope this will clear your doubts a bit