Here is the full list of all 34 3on3 FreeStyle achievements worth 1,000 gamerscore.
All Mission Clear.
Reach Level 20 (Max)
Win 300 Matches - "3vs3 Mode only"
MVP 200 times - "3vs3 Mode only"
10 Winning Streak - "3vs3 Mode only"
Reach Level 10
Win 100 Matches - "3vs3 Mode only"
MVP 50 times - "3vs3 Mode only"
7 Winning Streak - "3vs3 Mode only"
Win 10 Matches - "1vs1 Mode only"
Play 150 Matches total in any game mode
Score 3,000 points total in any game mode
Score 300 3PT shots total in any game mode
Get 500 Dunks total in any game mode
Get 1000 Assists total in any game mode
Get 500 Rebound total in any game mode
Get 300 Blocks total in any game mode
Get 100 Steals total in any game mode
Get 500 Loose Balls total in any game mode
Reach Grade 5 with any character (First Time)
Reach Level 5
Win 10 Matches - "3vs3 Mode only"
MVP for the first time - "3vs3 Mode only"
3 Winning Streak - "3vs3 Mode only"
Get a winning Buzzer Beater
Play a Match after forming with two friends - "3vs3 Mode only"
Score 1,000 points total in any game mode
Play 20 Matches total in any game mode
Lose 50 Matches
Use Substitution 50 times
Finish a Match with JOEY
Unlock 6 characters
Use Game-point to buy item at the Item shop for the first time