tell the differences between these words

Contrast refers to the differences between two things or ideas.

Condense means to reduce, to make smaller.

Compress means to apply pressure to make something fit into a smaller space.

Contrast and condense could be used to describe how one deals with an idea. Compress almost always refers only to a physical process, something which one does to a thing. One notable exception is the use of the term to describe how data is fit into a smaller space on a computer's hard drive--to compress data. The data is not made smaller, it is simply packed together in a more efficient manner.