At the Frigid Freeze Cave, there's some pretty tricky star coins.
One is at the bottom of the Underground Town, where there's a coin above some weird-looking terrain. Collect the coin, then you're golden.
The other one is at the top of it, above the last noteblock and to the left of the Shine Sprite. There's a ledge you'll climb up.
The last star coin is to the upper-left of the shard shine. Triple jump off the mushroom.
Here's pictures:
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Hope you do well with Pastel Citadel (and everything else)!
(the i's are meant to be separators)