difference(){
union()  {
cube([10,20,40]);
translate([10,0,0])cube([10,20,30]);
 translate([20,0,0])cube([10,20,20]);
  }

 
       translate([-1,10,10])rotate([0,90,0])cylinder(h=45, r=2, $fn=20);
  translate([-1,10,25])rotate([0,90,0])cylinder(h=45, r=2, $fn=20);
   translate([-1,10,35])rotate([0,90,0])cylinder(h=45, r=2, $fn=20);
}