When we pick the first computer, there will be a 3/20 chance of picking a defective computer. After this, we will have taken away one computer, and one defective computer. This will leave us with 2 defective computers and 19 computers.
If we draw a computer from this, there is a 2/19 chance of picking a defective computer. We repeat the same steps again: take away a defective computer and normal computer.
This leaves us with one defective computer and one normal computer. There is a 1/18 chance of drawing the defective computer.
To finish this problem, we multiply our three chances together. This is:
3/20 * 2/19 * 1/18 = 6/6840 = 1/1140, so the third answer is correct.