Here is the full list of all 50 Monster Energy Supercross - there are The Official Videogame 3 achievements worth 1,000 gamerscore.
Complete a Main Event in a Custom Track created by another player
Validate a total of 5 Custom Tracks
Earn a total of 1,000,000 SX Credits
Get a Holeshot in any game mode
Get a total of 15 Holeshot in any game mode
Win a Main Event with the Event Type option set on "The Real Thing"
Purchase a total of 10 components for your Rider Customization
Win a 450 Championship in Career mode
Win a 250 West Championship in Career mode
Win a 250 East Championship in Career mode
Reach the podium in 100 Main Events in any mode
Reach Prestige level 22
Reach Prestige level 70
Reach Prestige level 99
Create a rider with a female body type
Win a Team Day
Meet a Contract Objective of an Official Team in Career mode
Complete a full 450 Season in Career mode without changing Team
In a 450 championship in Career mode, sign a contract with an Official Team
Create a rider with a male body type
Pass at least 10 trials of the Challenge mode with any result
Complete all of the trials of the Challenge mode with the top result
Ride 100 miles (160 km) in offline Free Roaming in the Compound
Win a Main Event in Anaheim 1 in any game mode
Win a Main Event in Anaheim 2 in any game mode
Win a Main Event in Houston in any game mode
Win a Main Event in Glendale in any game mode
Win a Main Event in Oakland in any game mode
Win a Main Event in San Diego in any game mode
Win a Main Event in Arlington in any game mode
Win a Main Event in Detroit in any game mode
Win a Main Event in Atlanta in any game mode
Win a Main Event in Daytona in any game mode
Win a Main Event in Nashville in any game mode
Win a Main Event in Indianapolis in any game mode
Win a Main Event in Seattle in any game mode
Win a Main Event in Minneapolis in any game mode
Win a Main Event in Denver in any game mode
Win a Main Event in East Rutherford in any game mode
Win a Main Event in Las Vegas in any game mode
Unlock at least 3 Presets from different Official Teams in Career mode
Reach first place in a Bootcamp race
Save a custom Bike Setup
Change the colour of the nickname on the suit of your custom rider
Complete an Official Championship of any class in Championship mode
Get a Holeshot and victory in the same Main Event in any mode
Complete a total of at least 100 scrubs
Beat the time of the Ghost of another player in Time Attack mode
Complete at least two SX Challenges of the SX Profile
Reach at least fourth place in a Last Chance Qualifier in any game mode