I'm really confused about how to use "At", "In" and "On"

Could you use some examples? Please

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"He is at the bar" o "He is in the bar" Both are fine; 'at' is a bit more natural.

"Good luck in your exam" or "Good luck at your exam" The best word here is 'on,' but all three of them are okay?

"See you on august" or "See you at August" Both of these are unnatural; use 'in.'

"See you in the classroom" or "See you at the classroom" Go with 'in' for this.

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"At" gives a general location. For example, you can tell a friend that you're at the bakery. You might be inside of the bakery, or in the bakery's parking lot, or standing on the roof of the bakery. "At" is vague enough to be true for any of those. But if your friend wants to find you, now he knows to go to the bakery.

Time is sometimes 'in' and and sometimes 'on.' You would use 'on' in "See you on Monday," or "See you on Christmas" (days of the week and specific dates). You would use 'in' for "See you in August," or "See you in 2021" (months and years).

In also has some other abstract uses, like "in love" or "in the rain."