Here is the full list of all 26 Nickelodeon Kart Racers 3: Slime Speedway achievements.
Buy everything in the shop
Beat all 36 Ghosts in Time trial
Win 10 arena matches in any mode
Beat 8 Ghosts in Time trial
Get a blue-spark drift boost 21 times in cups
Perform 250 Slime stunts using wave edges
Complete all challenges
Buy 5 different things in the shop
Play 20 local multiplayer races
Complete 50 Double Dares
Hit opponents 500 times with items
Unlock 15 crew members
Unlock 5 chiefs
Achieve the "Slime legend" score in Slime sliding 20 times
Get 150 Victory Points competing in Online Matchmaking
Get 100 Victory Points competing in Online Matchmaking
Perform 250 Slime stunts
Drive through a Kart Wash 30 times
Play online with at least 3 other players
Get 50 Victory Points competing in Online Matchmaking
Complete a race without getting hit once
Drive through 50 boost pads
Play an online race with a friend
Complete your first cup
Finish a cup in first place
Finish a cup in first place 40 times