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