Here is the full list of all 34 3on3 FreeStyle: Rebound achievements.
Unlock the sixth character (Have Six Characters)
Get any item at the shop for the first time
MVP for the first time - "3vs3 Mode only"
3 Winning Streak - "3vs3 Mode only"
Play a Match by making team lineup with two users - "3vs3 Mode only"
Finish a Match with JOEY
Collect every trophy in 3on3 FreeStyle:Rebound
MVP 200 times - "3vs3 Mode only"
Win 300 Matches - "3vs3 Mode only"
Reach Level 20
10 Winning Streak - "3vs3 Mode only"
Win 100 Matches - "3vs3 Mode only"
Win 50 Matches - "1vs1 Mode only"
7 Winning Streak - "3vs3 Mode only"
Reach Grade 5 with any character
MVP 50 times - "3vs3 Mode only"
Reach Level 10
Score 3,000 points total in any game mode
Play 150 Matches total in any game mode
Get 1000 Assists total in any game mode
Get 500 Rebound total in any game mode
Get 500 Loose Balls total in any game mode
Get 500 Dunks total in any game mode
Get 300 Blocks total in any game mode
Score 300 3PT shots total in any game mode
Get 100 Steals total in any game mode
Win 10 Matches - "3vs3 Mode only"
Win 10 Matches - "1vs1 Mode only"
Lose 50 Matches
Play 20 Matches total in any game mode
Score 1,000 points total in any game mode
Use Substitution 50 times
Reach Level 5
Get a winning Buzzer Beater - "3vs3 Mode only"