How to obtain experience
You get experience by playing the game. After every match, you will earn a bit of experience. If you won that game, you would get more experience than if you lose the game. There is a cap on how much you experience you can get from any one event but the cap is fairly high and should not be a problem.
What to do with experience
You can’t spend experience and it just stays with you. the more experience you collect the higher your level. Three is not really a purpose for experience except to track how much you play.