Solved

Level up

You gain XP every league race you compete in.

The formula is 40 + ((Highest level in the tier - Your level) x 20)

As you are L3:
If the highest level team in your tier is 6 you get 40 + ((6-3) x 20) = 100
If the highest level team in your tier is 12 you get 40 + ((12-3) x 20) = 220 and
If the highest level team in your tier is 20 you get 40 + ((20-3) x 20) = 380

So to level up quickly you need to find a league that races every day with high level managers in your tier.