You can use then in many different situations, for example...

In: for spaces and specifics moments during the day, also to talk about months:

"I'm in the kitchen (space)."

"I'll see you in the morning (moment during the day)."

"I like the weather in January (month)."

On: for surfaces and to talk about a date or a moment day in a (general):

"The book is on the chair (surface)."

"My birthday is on April 1st (date)."

"I go to the movies on saturdays (day)."

At: for places and for time (specific):

"I'm at home (place, really general)."

"The concert starts at 8.00 p.m. (time)."

There are more uses for these prepositions though...

Espero ter conseguido te ajudar um pouquinho! :)

Hi Omar,

This is such a common question that it has been answered hundreds - maybe thousands - of times already.

Have a search for "difference in on at" and you'll find exactly what you're looking for.