If you have a Adafruit Motorshield v2, then this is the page to start: https://www.adafruit.com/product/1438
There is a link on that page to a tutorial. Go through the whole tutorial: https://learn.adafruit.com/adafruit-motor-shield-v2-for-arduino.
Start with a single motor and have a look at the examples at Github. For example this one: https://github.com/adafruit/Adafruit_Motor_Shield_V2_Library/blob/master/examples/DCMotorTest/DCMotorTest.ino
If that works, then you can try with three motors.
The difference between a beginner and a advanced Arduino user is that a beginner tries to rush to the end result, and a advanced Arduino user does a project step by step.