If you box 3 horse in a Quinella you place three bets: only one of these can collect, two must lose. What you are doing is betting against yourself. Dutch books are the same thing, when you bet more than one horse to win or three horses to place you are accepting that you have just made a losing bet.
However, the psychology of punters mean we like to win often and lack patience so rather than bet six trifectas on six different races we box three horses in one race. In the first instance it is possible that all six can return a dividend but when you box a trifecta you can only collect on one unit.
Don Scott recommended putting multiple trifecta combinations in a race but I suspect the overlays in those days were greater than now and he could afford to waste money on losing combinations.
Please correct me if I'm wrong, it does seem such a simple fact that I'm surprised so many people overlook it every day.