REQUIREMENTS FOR 3 STARS
Slippery Slalom = 01:12:99
Avalanche Ambush = 01:29:99
Snowglide Run = 01:16:99
You should be aiming to collect as many as the crystals as possible as these give you short speed boosts, following their line generally shows you a good path to take around rocks and jumps too.
Due to the very inconsistant snowbike it is advised that you avoid as many jumps as possible and try to not hit any rocks or make any sudden harsh turns.
I also found that pressing B (to jump) when going off a ramp only increases your chances of being wiped out upon landing.
I am currently unsure if bike enhancements from the main story carries over to the time trials, but I got the achievement without any upgrades so they aren't necessary.
EDIT: A lot of people are finding it easier to control the snowmobile when in first person view, Personally I preferred the widier vision but if you're struggling then this could be something to look into.
Feel free to leave further tips in the comments!