When you print an instance like house[0] in your example you see the result of the toString method. If in your class House toString isn't overriden you see the default implementation which displays the classname and the object-ID of the instance. So if you want some custom string representation of your House instances you can override the toString() method in class House. There you can e.g. display the values of the member variables, etc.
your for loop iterates from 0 to 2 so your i assumes 0,1,2 and your System.out statement prints i+3 which results in 3,4,5
