Raspberry Pi OpenCV 3.4.15
I did change a build script for OpenCV 4.x for the Raspberry Pi to build the 3.x version of OpenCV:
Raspberry Pi build script
Here is the homepage of the original script:
Original scripts homepage
The 3.4.15 version built on my Raspberry Pi 3 without problems. On the scripts homepage there is a tutorial too. It shows how to increase the swap file size, which is important! So you have to do this before running my script!
Have some fun!