Kylie Jenner -- My First Car Crash

She's only been on the road for 18 days, but Kylie Jenner already has a 3-car chain reaction collision under her belt ... TMZ has learned.

16-year-old Kylie -- driving her brand new $125k Mercedes SUV -- was driving in Calabasas yesterday around 5pm when she slammed into the back of a Toyota, which slammed into the back of a Subaru SUV.

The Toyota suffered major damage to the back-end of the car (you can see the back bumper stuck in Kylie's front grille). The Subaru also suffered bumper damage.

We spoke to a witness who says Kylie immediately got out of her car -- apologized -- and asked if everyone was OK. She eventually exchanged info with the other drivers and left.

Cops arrived to the scene a short time later, but Kylie was already gone.

We're told it didn't appear anyone suffered any serious injuries ... and everyone involved in the crash acted very cordial toward each other.

Kylie turned 16 on August 10 ... and was first spotted behind the wheel of the SUV at her Sweet 16 party on August 17.

Hopefully, the Jenner family has good insurance ...

12:08 PM PT -- Kylie's car is now at the body shop!