If anyone is still having problems with OpenCV 3 and Python 3.5, the easiest way is probably to go with Python's Anaconda distribution:
First, follow the platform-specific installation instructions on the website above.
Once Python is installed, go to a terminal and run:
$ conda install opencv
If you get a "package not found" error message, you can search anaconda.org
for packages that match your platform:
$ anaconda search -t conda opencv
In the list, pick a user that has the OpenCV version you want for the platform you want.
For example, OpenCV 3.1.0 for 'linux-64' and 'osx-64' is supplied by user `trung`, so simply run:
$ conda install -c trung opencv3
To show more details about a specific package, use:
$ anaconda show trung/opencv3