You can unlock Scrubs in Mad Skills Motocross 3 two different ways. 

1. Enter the MX Factory Challenges mode from the main menu. Then find and tap on the "SCRUBS" Challenge card. You will get a pop-up notifying you that scrubs are unlocked with a short video on how to scrub. 

2. Reach the "SCRUBS" MX Factory Challenge card in Career mode (Arizona). Once you reach this card you'll unlock Scrubs and get a pop-up explaining how to scrub.