I am assuming you are just making a triangle, not a right-angled triangle.
Think of these as two separate pieces of wood. The largest possible third side would be the two pieces of wood end to end, with them just a tiny tiny tiny bit less than 180 degrees, nearly forming a straight line. In this case, the third side would be just under 22 units long. So the largest whole-numbered length would be 21 units.
Hope this helped! Have a great day!
