So I think this can be earned on either the cop or the racer mode, but cop is generally easier (I hear) so start with that one.
To be honest, you're not going to get 3 stars on all the events in tier 1 right away. You'll need to progress and get some extra stars in tier 2 and maybe tier 3 before you have a car that's good enough to 3-star all events in tier 1. I was cop level 7 or 8 before I could 3-star all the tier 1 events.