Great news everybody. I have upgraded my OSMC to nightly build by @gmc Kodi 18 (Leia) builds for Raspberry Pi and now I can play netflix on my RPi3 OSMC install. All necessary information and netflix addon can be found on Netflix addon github.
DISCLAIMER: This includes upgrading OSMC to nightly build. There may be very ugly bugs. If you don’t know what you are doing, wait for the official Leia 18 release.
There were only two things I had to do differently.
(first) When downloading widevine library I had to modify the getwidevine.sh script. Change this line:
[ -z "${ARCH}" ] && ARCH=$(grep -m1 ARCH= /etc/os-release | sed 's/"//g' | cut -d. -f2)
to
[ -z "${ARCH}" ] && ARCH=arm
Then run the script
sh getwidevine.sh
(second) The second thing is because of missing cryptodomex library in OSMC. Therefore we need to install python-pip through ssh and then install cryptodomex
sudo apt install python-pip
sudo pip install pycryptodomex
After all this (and changing max resolution in inputstream.adaptive to 480p - not sure if necessary) my son watches Peppa Pig from netflix right now 
Enjoy.
The picture was taken a few seconds before my wife discovered Peppa Pig