Hi and welcome
Marketfeeder can certainly help to automate what you describe, and a lot more. I would say from the start that there is no guarantee that the price of the horse you back at over 3 is going to come in below 2. The software and triggers will allow you to hedge your position for a profit (green-up), or close for a loss if the position moves against you (distribute loss).
writing triggers is basically a set of clear instructions, e.g.:
at 1 minute before the scheduled start of the race, back the second favourite if it's price is between 3.00 and 3.50
when the market goes in-play:
if the price comes in below 2.0, green-up for equal profit across all selections (guaranteed profit no matter which selection wins)
if the price drifts above 5.0, distribute loss equally across all selections (stop-loss)
in the attached screenshot I have converted the above to a trigger, hopefully this will get you started on trigger writing which is great fun.
There are loads of example triggers on the website:
http://marketfeederpro.com/learn/triggers/and if you want a trigger written for you:
http://marketfeederpro.com/learn/triggers/trigger-development-service/hope this helps...please use test mode for developing and testing your triggers before switching to real money.