I too have struggled getting OpenCV3 installed on my system. Fortunately, I found this youtube video to be extremely helpful.
I do not know for sure if it is specific to Python 3. But I do know that he was using python to test it out.
When you get to the point of compiling OpenCV from the source code, I would suggest using the cmake-gui
sudo apt-get install cmake-gui
The gui will allow you to see and enable/disable - graphically - all of the modules associated with OpenCV.
When configuring on the RPi, make sure you uncheck any references to
cuda. This is the only REQUIRED step for you to take to get the library compiled. But there are lots of other things you can add on.
NOTE: when you run
make It will take FOREVER on your PI. I would suggest running:
sudo make -j3
So that you run the compilation using 3 cores instead of 1. This will cut down the compilation from about 3-4 hours down to around 1-2.
I hope this helps...