This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The error message actually gives you a lot of information here.
Square is not abstract and does not override abstract method area(int,int,int) in Shape
That's the reason, right there. Square is not an abstract class. Therefore it must provide an implementation of all methods in interfaces it implements. You either have to implement area(int, int, int), or you need to mark Square as abstract - which means you're expecting subclasses to implement it.