I did this by having three contollers "plugged in" then doing
online>Invite only>7 AI(even though it will be 5)> Lap count 7
Then using RB you can set main to a "cop" all AI and dummy controllers to "robbers"
Hit the two dummy controllers at the start just like in "Seeing Double" achievement, then rejoin the AI once they have lapped you and convert any of them. You should be the only active cop so will not have any competition getting the third and final conversion.
Achievement unlocks as soon as you hit third car.