Squirrel #1: Waterworks
After jumping onto the platform over the water that falls forward when you land on it, reverse back up it to reach the ladder on the left side.
Roll backwards down the ladder until you reach the water level with your rear wheel, then ride back up it and you'll see a squirrel sign has been revealed.
NOTE: As long as you roll down to the bottom of the ladder, it doesn't matter if you crash and respawn at the checkpoint.
Now finish the track and the squirrel will appear at the far end.
Squirrel #2: Observatory
At the top of the ramp with red sides, slow right down and crawl past the 'Slow Down' sign until you hear a chime.
Stop before you reach the platform ahead that launches you upwards, and instead reverse back down the track to reveal a squirrel sign.
Keep rolling backwards down the hill until you find the squirrel and explosives that have appeared in the water.
Squirrel #3: Cliffhanger
After jumping over the canyon road near the end of the track, stop at the top of the next rocky ramp and roll backwards across the platform.
Drop off the platform and lean back to launch yourself onto the road below.
If you've landed in the right place you'll hear a chime, then wait until a cutscene kicks in.
A truck arrives shortly afterwards, delivering the next squirrel.