When you put 2 of the 3 items... it should autofill the 3rd...
So, if you put Quantity+Price... it should work out the total amount that you would need to have in your account to be able to buy that. If you put Price+Total... it should work out the quantity of BTC that you're going to receive. And if for some weird reason you put Quantity and Total, it will tell you the price