Easiest by far for me was TEST DRIVER: HORIZON RUSH CANYON CRUISE.
all 3 chapters have plenty of time to get you 3 stars each.
#1 canyon cruise.
a no brainer.
drive and take short cuts, crashing off cliffs etc etc.
it's what we do best.
#2 stunt master.
don't drive and drift.
you're already at the top near the end.
just drive around crashing into small objects, trees and guardrails.
build that multiplier up to 5X.
you can do the math.
#3 high jump.
just drive to and STOP at the last sharp curve.
press down on the Left Thumb stick
the cars exterior stuff will lower making you more aerodynamic.
you'll be near the guardrail.
gun it , you'll have more than enough speed.
you'll do a small bounce just before the ramp.
don't worry, hit the ramp right to left.
landing onto or to the left of the road below.
LEMON
PS: remember, the Y button
is your best friend.