Just thinking about this quickly, "comprised of" means that something contains something else. If you look for a apartment, say, "the apartment is comprised of one bedroom and a living room" for example. Whereas, "composed of" means that something is made up of something else. For example, "the cake is composed of eggs, flour and milk". Constitute has a similar, but not identical, meaning. That is to be a part of something else. For example, "England, Scotland, Wales and N Ireland constitute the United Kingdom" I realise though that this is probably still confusing....all I can say is that usage and practice will help you to decide when to use which of these expressions. You can take comfort in the fact that many "native" speakers of English misuse these expressions regularly :-)

To constitute means to be a part of a larger group. Ex. Bored students constitute the majority of the school's population. To be composed of means to be made up of. Ex. All humans are composed of individual cells. Ex. All salads are composed of lettuce, tomatoes, and salad dressing. To comprise means to contain or include. Ex. This activity comprises everyone. "Constitute" isn't used very often, at least in my experiences in American English. Most of the time, "comprised" and "composed" are understood to mean the same thing despite the fact that they technically have slightly different meanings, so I wouldn't worry about trying to use them correctly - most people won't notice the difference. In all of my examples for "composed," you could insert "comprised," and the sentence would still work.