They are playing "on" the beach.

1. Can we use "at" the beach or "in" the beach?

2. What are the differences between at the beach and on the beach?

3. Is in the beach right?

thank you
"At the beach" simply tells you where you are. "On the beach" tells you where you are and draws attention to the fact that you're on the sand.

You can't say In the beach because "In" means you're in somewhere, like "In a house" "in a factory". And beach is not something you can be "In" there right? So "in the beach is not right.

"at the", you can say " i work at the GM" or "i work for the GM" they're both ok. Or "I will wait you at the corner"
 MrPedantic's reply was promoted to an answer.