Multi pass rendering, although it is used in other scenarios, nowadays is typically used to describe a way to render stereoscopic images. Multi pass means that depth rendering and other initial calculations for rendering to each eye are done separately for each eye (more than one pass= multi). Some newer methods are more performant by doing a single pass on these calculations for both eyes at once.
One single pass is typically more performant than multi pass. However, this optimization comes with some hits to flexibility. It’s more difficult to do different rendering to each eye.
Post processing typically refers to anything you do to the rendered image after rendering occurs. So you could do post-processing on either multipass rendered images or single pass images. Some typical post processing effects include exposure, bloom, tinting, split toning etc. although there are a huuuuge array of different things you can do in post processing.
Compositing usually involves combining multiple rendered image to form a single result. For example you may overlay one rendered image on top of another. Portal effects typically use compositing. The area of the primary view image that is taken up by the portal is replaced by the image rendered from a secondary viewpoint that is viewing the scene at a different location. These two viewpoints are composited to create the resulting effect.