GRID Legends

EDIT: For Deluxe Edition owners, tier 3 will unlock at 56km./35mi. because of the mechanic pass which lower the requirements by 30%. This means that only 14 laps around the Oval is needed for unused cars to unlock tier 3.

Credit goes to PureRhapsody for the car list, I only added what class each car is in Race Creator.

There are 9 Ferraris you need to acquire and drive 80km./50mi. with to unlock the tier 3 upgrades, and the unlock requirement for each car can be viewed in the Garage tab in the menu. Most of them will be unlocked in the later stages of the career mode.
You can check what distance your car has travelled by pressing at the car select screen before a race, or when viewing the car in the garage. If tier 3 is not unlocked, go to the tier 3 upgrade and the distance driven will be displayed as X/80km. or X/50mi. Use this to calculate how many laps you need in Race Creator below. You DO NOT have to buy any upgrades for this achievement, only unlock them.

Car list in garage order and their class in Race Creator/Career:

GT class:
Ferrari 488 GTE (GT Class 2)

Track Day:
Ferrari 599XX Evo (Track Day Specials)
Ferrari FXX-K Evo (Track Day Hypercar)
Ferrari FXX (Track Day GT)

Specials:
Ferrari F40 LM (Vintage GT)
Ferrari 512 BB LM (Classic Ferrari Cup)
Ferrari F430 Challenge (Ferrari Cup)
Ferrari 365 GTB4 Competizione (Classic GT)
Ferrari 330 P4 (Historic GT)

Try to unlock each Ferrari as soon as you can and then use it in its championship, as you need to complete most of them anyways to unlock The Gauntlet. Once each Ferrari has no more career events/you have already unlocked The Gauntlet, head over to Race Creator and create an event with the following settings:

Modifiers: Boosts- Lap Boosts, this will save a few seconds each lap, use with (default)
Discipline: Circuit Racing
Class: see above which class you need here ;)
Vehicles: Make sure the Ferrari has a green check mark next to it, it's on by default
Location: Indianapolis
Route: Oval Circuit ( each lap is 4 km./2.5 mi. long)
Conditions: Day
Laps: You need 20 laps if you have never used the car before, otherwise calculate your missing km./mi. with the distance each lap has.
Drivers: 2
Advanced: Vehicles Upgrades on + rolling start, and any other setting you might like.

After hitting to create this event, all the settings should be saved and you only need to switch the car/class + amount of laps each time from now on. Remember before each race to tune the gear ratio to long for a higher top speed with the lap boost.

For every Ferrari you unlock tier 3 with, the achievement tracker will move up 11%, just double check that all of the upgrades are unlocked before moving on to the next car. This one should be an easy unlock, albeit a bit time consuming.