How can I encourage my 3yr old to talk voluntary on his own rather than him only speaking when he mocks someone

Personally, I'd ignore it. My two-year-old just learned my name and will call me that when I don't respond to "Mommy"; eventually she switches back to "Mommy" and then I make a big deal out of how excited I am to talk to her :)

Ask millions of questions when you are playing. Alphabets, numbers, animals, objects, etc. When you read books and do puzzles together, always ask him what things are. Is there anything he is really wild about? Trucks or dinosaurs etc? His favorite things may be the easiest to get him to talk about.