So your 2nd and 3rd panels has not been as bad as the 1st? Just to clear that up?
I have had a rMBP from early 2013 with an LG panel, and it suffered bad after just 2 months. I asked for a Samsung panel, and after almost 3 years, no more retention. So I an familiar with the problem.
I don't have AppleCare on my unit no.
My first screen was definitely the worst. My second screen had retention starting from day 1 though, which was odd. My third display has so far been the best, however has started to develop image retention, and from what I can tell, is on track to becoming as bad as the first screen.
Also damnit, you have no apple care. The EU has lots of regulations about 2, 5, 6 year warranty right? If you're in Denmark maybe you could look up some regulations, call Apple Care and try for a replacement anyway?