I think what you will find is that the races where the is 2nd fav <=4.50 will work strongest with what you are trying to do.

Also where the 1st fav is approx 2.50 & less.

Another way is add up the outlay for the 2nd & 3rd Fav
Then divide that figure by the Favs fractional odds to give us the amt to put on the Fav so it breaks even if it should win.

Example
Lay Bet 2nd & 3rd to say $30 Liab
2.88---O/L 9.22----Liab - break even
3.65---O/L 11.32---Liab - $14.65
5.90---O/L 6.12 ---Liab -$9.44

Total O/L $26.66

This makes the bet look like a 1/1 proposition as opposed to just taking the 2nd & 3rd fav on their own which would be double the Liab.

__________________
Cheers.