This is for winning 3 Races in a row as stated, this was fairly easy just playing at 75% difficulty, but you can simply lower it all the way, then set laps to 1 and race on a familiar map to win 3 races in a row. This can be done in Arcade or through Campaign progression. One final note is that vehicle upgrades will help in winning, although you should be able to do it fairly easy without any.