Answer 1

At sololearn 😄
But Udacity is the best place for online learning and Udemy also provide free courses and free certificate here's the name of free courses in Udemy:-
#Become an Android Developer from Scratch
#Deep Learning Prerequisites: The Numpy Stack in Python
#AWS Concepts
#Machine Learning
#Full-Stack Web Development with React
#The Complete iOS 10 Developer - Create Real Apps in Swift 3

Answer 2

IBM also provides free certificates https://cognitiveclass.ai/

Answer 3

I think we need SAS, VBA and R language too in Sololearn