may someone tell me where the heck am I supposed to find the entrance of this?
cuz I was trying to enter potion shop, but it says it still on development

You can access it from the overworld map via leaving the town. it's to the north on the overworld.

I'm inside Devil Flower Demon's Grotto, and I can't find the third gem to insert into the last pillar, I've turned the map of Devil Flower Demon's Grotto 10 times but I can't find it, could someone please help me?