There are several things to consider when choosing a language.
For speed of development, much depends on which language you have the most experience with already. A new language always has a learning curve at the beginning. But assuming that you are comfortable with all three, I would say that Python is probably the language someone could develop code the fastest in. One reason for this is that you do the development and debugging within the same IDE. You can also do some prototyping using the text window in Fusion. C++ has an advantage that it is typed so you're able to catch many problems while writing and compiling, while you end up debugging a lot of problems at run time with the other two languages.
If you don't care about the security of your code, I would recommend Python and if you do need to protect your code then C++.
I agree with Brian’s comments. Just one of my practices of Fusion API: I prefer to Python because it is easy to debug and make a quick demo code.