I am now planning to start with the third chapter of my novel. I got an interesting idea to write it as a ballad. But I am much better with prose. Will I better off if I continue writing prose or should I experiment with a ballad?
Are you on a deadline? If not, there's little downside to experimenting with a ballad form. Might be harder to get traditionally published, but still: I'd say if you've got the urge to give a ballad a shot, go for it. If it doesn't work so well nothing would stop you redoing it in prose. You could also do it the other way - keep going so you don't break the prose, but then have fun trying it out as a ballad after and see how you like it.
But I'd definately encourage giving it a shot.
Since you have written first two chapters in prose, you should stick with prose for them. However, as you have a strong urge to write a ballad, I would say that you must give that a try. If you could manage to write it successfully, you could start the third chapter in prose and then introduce the ballad in pieces along with the prose.