What you are assuming with this Rule , is that for some reason, the 3rd Favourite is overbet at Greyhounds.
And that it is in fact Overbet to the extent, that the 15% Takeout still ain't enough to overcome this massive betting splurge.

Unless you can come up with some sort of reason why the 3rd Favourite is overbet , you are doomed to lose.

And just another point, the 3rd Favourite will probably vary minute by minute due to the small size of the Pools.

I figure you either lack understanding of the basic principles of Gambling, or have not thought this through.

Or you simply enjoy gambling so much, that losing isn't an issue.
A bit like those Poker Machine addicts.

Please don't take this as a personal attack.
Much of the above applies to me.