According to what I read it is because of an odd implementation of that function, apparently the number is used to add a Limit statement to the SQL that is used to get the pages. Then it applies the child_of requirement after retrieving the data from the database. So if you limit the query to 3 chances are you will not get any results that are also a child_of your page.
Instead leave the number parameter off and do the limit in your foreach block. Ie increment a variable and if it is greater than your limit, exit the for loop.