Study participants reported increasingly favorable impressions of a cereal, a restaurant, a shampoo, an ice cream store, and a politician as researchers raised the number of positive claims about each one, but only up to a point: Beginning with the fourth positive claim, participants’ impressions became less favorable, say Suzanne B. Shu of UCLA and Kurt A. Carlson of Georgetown University. This and other experiments demonstrate that when four or more positive claims are made about a product (or a firm or an individual), people infer that the claimant is trying too hard and may have ulterior motives.
Source: http://www.anderson.ucla.edu/faculty/suzanne.shu/Shu Carlson Three in Persuasion.pdf
To Sell Your Product, Make Three Claims, not Four