Then with a summary the merge should work. I do not use merge as I have never found it to really work as well as I would like. They may need to all enter into the same fields otherwise, even if you label them differently for each. Merge wants to make a repeating node of the similar data so I am not sure it will work with how you have it setup.
Otherwise have each one update the same form instance. Like a simple workflow and then all the related data would be in one form. This is much simpler and easier to implement. You can logic to prevent each one from seeing the other section if necessary.
