If you are advanced enough in English to be worrying about these 3 words, then I think you are very capable of looking up the definitions in a dictionary and working it out yourself. Though, seeing as you asked... A proponent is a person that proposes something or makes a proposition, an advocate argues in favour of something (possibly publicly). Though both words might be used to refer to someone that is in support of something. An approver is a person that exercises the right to allow something to go ahead. These are big words, with maybe more than one clear definition, so I would look them up yourself, along with finding examples of clever people using them. They sound like words lawyers might use a little more frequently than your everyday folk.