Because you live in Obama's socialist Unova, you can only ever possess three Pokemon at a time. You can have them for whatever reason (battling, companionship, contests, protection, etc.).

So what three do you pick and why?

Scrafty - Cool mohawk lizard, able to gift me small stolen objects / beat up my adversaries.
Scolipede - A friggin' poison bug-horse. Simply majestic.
Emolga - Cute little flying rodent. Girls love these sort of things.