Could you tell me what is the difference among trash, junk, garbage?

Garbage is what you put in the trash can/rubbish bin. Stuff from your kitchen, used tissues, etc. plus anything else you throw away. I don't see much of a difference between garbage and trash.

Junk, on the other hand, is "stuff." Some people will tell you that junk is just trash that you haven't thrown out yet. However, junk would NOT include kitchen waste or tissues, etc. It could be anything from a broken trumpet to a kitchen device you never use.

You can walk into someone's attic and say "Wow, look at all this junk!" Don't use "trash" in that context. On the other hand, you walk into an empty lot that people have been leaving their garbage in and look at the empty soda bottles and wet cardboard and dirty diapers and say "Wow, look at all this garbage/trash." Don't use "junk" in this context.

In fact, I used "trash can" in my post above.

I think I use them interchangeably. There's a little trash can in my bathroom, and a larger garbage bin in my kitchen. There's a huge garbage can outside my garage that gets toted the curb on trash day. Any of these could mixed-and-matched.

There are some differences, I believe, in how it's use in BE, so we'll wait for Nona or Mr. P. or another Brit to come by.