A wireframe is a outline sketch of layout. It doesn't look anything like the real finished product. It just shows where the various elements will be.
A prototype and a mock-up....there we don't have exact definitions and ways of differentiating them.
I would say that "mock-up" to me sounds like something that is still in fairly early stages whilst a "prototype" is almost complete, a beta version almost. However this is just a feeling and generally the two words are fairly interchangeable, prototype especially can be at either end of the design process.