Make sure you have official WhatsApp installed in your phone and its setup with your 1st sim.
Now Download the apk.
You may get unknown source warning so just go to settings and turn on unknown sources.
then install the GB WhatsApp it will be installed.
Open GB WhatsApp and enter your 2nd number here.
WOOH! You just did it.Now your phone is running 2 WhatsApp simultaneously.
If you want to use 3 WhatsApp.
Then download Disa app from play store.
Open the app.
Skip the intro.
Click on add service.Download WhatsApp plugin.
Now enter your name, Your 3rd mobile number with country code.
Verify it by SMS or call.
Congratulations now you have 3 WhatsApp running on a single android phone.
This tutorial will work with any android phone root access is not required.