Team racing is most often three boats against three boats. Low point scoring is used and your team's score is the total of each boat's finish. With six total boats on the race course the total number of points at the finish will be 21 (presuming no boats will be disqualified after the race). Therefore, to win a team race your team needs to score 10 or fewer points.
1, 2, 3
1, 2, 4
1, 2, 5
1, 2, 6
1, 3, 4
1, 3, 5
1, 3, 6
1, 4, 5
2, 3, 4
2, 3, 5
4, 5, 6
3, 5, 6
3, 4, 6
2, 5, 6
2, 5, 6
2, 4, 6
2, 4, 5
2, 3, 6
1, 5, 6
1, 4, 6
You'll notice that in 3v3 team-racing all the winning combinations add up to 10 points or less. All the losing combinations add up to 11 points or more. There are 21 points on the race course (1+2+3+4+5+6=21) so 10 becomes the point threshold for winning.
Rather than memorizing ten winning combinations, you can simplify the combinations in the way shown below. This is what team-racers refer to as "plays." Calling a play on the race course is a way of communicating to your team that you are either in that winning combination, or should be working to get into that winning combination. The "x" in play one indicates that your third boat could be in any of the remaining positions.
Play 1 and Play 2 are stable winning combinations. This is because two of your boats are beating two of their boats while the same is not true for the other team. As a team you are in control of your own destiny.
Play 4 is an unstable winning combination. While two of your boats are beating two of their boats, two of their boats are also beating two of your boats. That means that with effective team racing the team in the losing combination (2,3,6) can convert to a winning combination through their own actions. So while play 4 is winning, be careful because it is an unstable winner. You might want to convert to Play 2 if the opportunity presents itself.
There are a number of other winning combinations, but the goal is to get into one of the plays above (preferably play 1 or 2) as they are stable winning combinations.