Competitive Tournaments pit you against teams of similar skill to earn rewards. You earn Tournament Credits when you finish a Competitive Tournament match. You can also earn bonus Tournament Credits for your Top Three Weekly Placements.
The counter resets every Sunday. Your first three matches count towards your Top three Weekly Placements no matter if you win or lose. You will only get rewards if you win at least one of the matches.
Once you have played your first three Tournaments, you can earn additional Weekly Placement Rewards if you place higher than your previous Weekly Tournaments. The more you play, the more you earn!
For more info, check out: Revamped Tournaments: A Closer Look