Excel formula question

Is there any formula which will look at a range of cells (couple thousand long) and the result of the formula will be the display of the terms that occur the top three times. Not the count of occurences but the actual terms. I have a list of ad sources and would like be able to analyze the list and know which sources appear are the top three performers. The top three could change week to week so I am not able to count the occurences of a certain adsource because I don’t know what the top ad source is week to week. Any help is greatly appreciated.