Which 3 Unique Upgrades (TJ parts) are the best?

The three most effective TJ setups for NFS Underground, especially in Circuit and Free Run mode, are:

EngineECUTurbo→the strongest option, but slightly less stable and a bit harder to handle.
EngineTurboWeight→slightly less power, but easier to control and more stable.
EngineECUWeight→similar to Engine-Turbo-Weight in handling and stability.

Try all three and choose the one that feels best for you - you really can’t go wrong with any of them.
You can always change the TJ combination in the NFS.onl launcher under Options → Saved game files, as long as the game is not running with that profile loaded.

All Upgrades

CodeUpgradeShort Name
ENGEngine Exhaust PackageEngine
TRNDrivetrain PackageTrans (for Transmission)
TIRTyresTires
ECUE.C.U. & Fuel SystemECU (for Electronic Control Unit)
TRBTurbo PackageTurbo
BRKBrake KitsBrakes
WGTWeight Reduction KitsWeight
SUSSuspension PackageSuspension
NOSNitrous Oxide PackageNOS or Nitro