Can someone tell me the ways of getting free membership (gold, platinum or diamond )?

Occasionally there are contests that might award free memberships but beyond that, there really isn't a way. 

You can bribe me  

You can wait until Erik hosts another of his periodic "state of chess.com" TV shows and ask a controversial question in the chat.  Then after he makes fun of you, quit the site, and give Erik's public bullying as a reason.  Hey, it worked a year of free Diamond for me...

stand and deliver JD, this is a stick up, make with the diamonds!

Me and my friend helped JD fix an interesting bug. Should have gotten at least a month of gold

Go to any major intersection with a cardboard sign that says "Need Chess.com Diamond Membership." Smear dirt on your clothes and your face. I'm sure some motorists will help you out . . .

if I got diamond I would give it to someone that needed or wanted it.