Ok, I understand what you mean. That will be kind of hard to do. You would have to check if the combination of numbers that are on the screen can add up to the target-number.
I'm not a math-wizard, so I can't really help I think.
Maybe you could turn it around. pick three instances at random and add up their animationframes and set that number as target. Than you could keep the rest of the events as is..
I told my dentist I had trouble with my teeth and asked her to fix it without looking in my mouth..