Backing the outsider of 3 is a myth. Less than 20% in any given season, and you can use this to your advantage by laying the outsider in every race of 3 runners. This will guarantee long-term profits, but avoid those offered over 16.0 on the exchanges.
They only win around 15% of the time and are bad value. Don’t expect to get rich quick though as you are only looking at around 10pts profit a year, but every little helps!