Hi. I've been reading this tread since very beginning. I'm not a pro, I am just interested in general aviation and technical stuff. I just thought I register here to help you clarify some doubts you've been having regarding the info available in Polish media.
You've been asking what is the third recorder that has just been found.
Well, according to Polish media, they all say that the 3rd recorder registers "podwyższone" flight parameters. The problem is "podwyższone" doesn't seem to be a technical term but rather some sort of miss-translated Russian/technical jargon. The word "podwyższone" literally stands for "elevated", but it may also mean improved/refined/extended/advanced... Maybe that will point you in the right direction.