// a little cube with x = 30, y = 20, z = 10
x = 30;
y = 20;
z = 10;
offset = 4;

union()
{
    difference()
    {
        cube([x, y, z,], center = true);
    
        cube([x - offset, y -offset, z+offset], center = true);
    
        cube([x + offset, y + offset, z - offset], center = true);
    }
    
    sphere( r = offset, $fn = 128);
}