If you mean your world should be just a 3x3 grid you could just adjust this in the World-class:
public class MyWorld extends World
{
public MyWorld()
{
super(3, 3, 200);
}
}
This means that your world is consisting of 3x3 cells, every cell is 200x200 pixels (so the width of your world actually is 600px - 3 cells a 200 px)..
You can set your marks with something like
// in World-class
Cross m = new Cross();
addObject(m, 1, 1);
This sets a mark into the middle cell, as you start counting cells with 0:
0/0 1/0 2/0
1/0 1/1 1/2
2/0 2/1 2/2