Which prepositions should I need to use when giving an exact time and location?

Let's meet at the cafe at 3. (correct and normal)

Let's meet at 3 at the cafe. (correct and normal)

You may change the preposition according to the place and in fact it is usual in some cases to use another one than "at". If the precise spot is near the place and not in, the choice is not "in".

A city — "in" prefered

• Let's meet at 11 in Atlanta (ngram)

A public park — much more often "in"

• Let's meet at 11 in Central Park. (ngram)

Park — "in" or "at"

• We met at 11 in/at a park (ngram)

train station — "strictly "at"

• Let's meet at 11 at the train station. (ngram)

A restaurant — "at" preferred but you can say "in"

• Let's meet at 11 at/in the restaurant. (ngram)

School — "at" or "in"

• We met at 11 in/at school. (ngram)

Stadium — "at" much more common than "in"

• Let's meet at 11 at the stadium. (ngram)

River side — "by" is usual, "at" is not found and although it seems a reasonable possibility, it is not found.

• Let's meet at 11 by the river side. (ngram)

Hill — "on" common, "at" rarer

• Let's meet at 11 on/at the hill. (ngram)

Cafe — "at" preferred but "in" also used

• let's meet at 11 at/in the cafe. (ngram)

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