I'm in/at/on

Most commonly, you would use the word 'in' when entering a physical location such as a room or a building.

Here are some examples:

"I'm in the shower."
"I'm in the lobby."
"I'm in a car."
"I'm in a house."
"I'm in a school."

Using the word 'at' helps tell someone where you currently are. The difference between 'at' and 'in' is that the physical location is general.

Here are some examples:

"I'm at the grocery."
"I'm at the mall."
"I'm at the doctor's office."
"I'm at the park."
"I'm at the airport."

However, in some cases you can use 'at' and 'in' interchangeably.

Here are some examples:

"I'm at the mall."
"I'm in the mall."
"I'm at the park."
"I'm in the park."
"I'm at the grocery."
"I'm in the grocery."

Using the word 'on' is referring to a non physical location such as your time being utilized by something else.

Here are some examples:

"I'm on the phone."
"I'm on my computer."
"I'm on a bus."