3 blocks of carbohydrate does not mean just 27g of vegetables. What it means is enough vegetable to get you 3 blocks of carbohydrate.
For e.g. 3 cups of Cabbage or 4 cups Cauliflower pieces make just 1 block of carbohydrate where as just 1/4 cup of Chickpeas will also make 1 bloc.
Plz download the pdf file to get the details about different food items wrt blocks . . http://www.drsears.com/ArticlePrevi...fault.aspx