Join all of the three tournaments by selecting "Online" in Mario Kart 8 Deluxe, then select "Tournaments" and "Search by code."
In order to compete in the online tournament, you must have a ™Nintendo Switch, Mario Kart 8 Deluxe and a Nintendo Online account.
Information on how to sign up for ™Nintendo Online: https://www.nintendo.com/switch/online-service/