How to solve my problem

High=5.00;
Outer_radius=19.5;
Inner_radius=12.00;
Radius_drill_hole=1.5;


difference() {
	cylinder (h=High,r=Outer_radius,center=true);
	cylinder (h=High,r=Inner_radius, center=true);

	difference() {
    	rotate ([90,0,0]) cylinder (h = Outer_radius*2, r=Radius_drill_hole, center = true);
	}

	difference() {
		rotate ([0,90,0]) cylinder (h = Outer_radius*2, r=Radius_drill_hole, center = true);
	}
}

Quote
rhmorrison
3mm threads are pretty small for an FDM printer (if that is what you are using).
For those holes I would recommend making a hexagon shape of 2.75mm and then threading the holes manually.