ImportError: libcblas.so.3: cannot open shared object file: No such file or directory

What worked for me (I was missing some dependencies):

pip3 install opencv-python 
sudo apt-get install libcblas-dev
sudo apt-get install libhdf5-dev
sudo apt-get install libhdf5-serial-dev
sudo apt-get install libatlas-base-dev
sudo apt-get install libjasper-dev 
sudo apt-get install libqtgui4 
sudo apt-get install libqt4-test

Exact same solution as @thvs86 but here's a single 1 line copy-paste so you don't have to insert each command individually:

pip3 install opencv-contrib-python; sudo apt-get install -y libatlas-base-dev libhdf5-dev libhdf5-serial-dev libatlas-base-dev libjasper-dev libqtgui4 libqt4-test

You need to install only one package with neccessary shared object for it to work

sudo apt-get install libatlas-base-dev