Hi,
I'm not from Welldone, but many problems are solvable by the forum users.
Your trigger is overcomplicated but it is a good first effort.
I have amended your trigger. It now will do the following
A) 5 mins to the off, back any horses priced between 3 and 7 at the current back price.
B) Lay the backed selection to an amount for 3% profit, but you can that to any % you want.
Test it first and good luck
Larp