After rereading your post it sounds like cross chaining. Don't use big rear sprockets with the big chain ring. You are forcing the chain to bend laterally whenever you cross chain and big sprocket to big chain ring forces the chain to bend the most as the distance between the two points at which the chain bends is the shortest.

You need to use the middle chainring and a smaller rear sprocket. It will give the same gearing and a straighter chain.

Using a triple exacerbates the problem as the big chain ring must be further out than if you were using a double chain ring setup.

EDIT: a new chain will also bend side to side less than an older worn one.