How to merge 3 courses and transfer all training records

I have many questions…but in theory you would need to transfer completion dates to training material completion dates….so I think the theory is sound. Because you are doing a containership shift - you should consider what it means to also mark the course as completed. 
The containership of your objects may not be ideal though so consider if you still want to report out on the completion records within that one “new” course container. In theory the training material can be given a time/date stamp…but its “reportability” via Reports/New Reports tools is only at the course level. Just by that alone you may want to change your containership plan to Learning plans / Courses…both reportable via those tools.