Calculate the book % in the 3 favourites

Hello everyone.

I have a dumb question but I couldn't find topic regarding this...

I want a trigger to make a back dutching on the first 3 favourites in a horse race, if the book % of those 3 favourites is equal or greater that 90% (and less than 100%)... And a greenup when the profit is greater than 10% of the total stake (or liability, for Lay Dutchings)...

My main problem is to calculate the book of the 3 favourites....

Can anyone point me a direction on this?

Thank you in advance for any help you can give...

Hi
Try this:
selections rank is between 1 and 3
and selections trigger expression matching_number is equal to 3
and selections trigger expression match_b_book is equal or greater than 90

matching_number is the number of matching selections of previous conditions 
match_b_book is the back book percentage of matching selections

if you wanted the lay book percentage of matching selections it is match_l_book