also see the first answer here
and they are usually all interchangeable when referring to something being behind. So it very much depends on the context.
They can all be interchanged if referring to something being behind something = something or someone else is in between you and the object/building scenery etc..
"in the back of". "in the back". and "at the back." Would be used to also say that something or someone is in a space or place furthest away from the front of the space.
This is a question that would easily be explained with pictures or flashcards something italki does not provide unfortunately :))))