We’re dishing out the rewards based on the tier you reach at the end of Season 3 ranked play. You’ll earn rewards by playing ranked in any game mode (solo, duo, 5v5 or 3v3), and you’ll nab the reward corresponding to the highest tier you hit across all modes. Keep in mind that for 5v5 and 3v3 tier rewards, you need to have won at least 10 games with the team to qualify.

No point in buying a ranked team slot if you're not going to be able to win any games in that tier, it's going to be pretty hard for you to win any games in Diamond if you're Silver. Good luck anyways if you are buying it
It's just a warning.