Found two of the three exits, can't figure out the last one, I'm guessing it's the hardist.
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I don't want to say how to do it in the comments, but I will say that I knew what the c wall would be right away and looked for it in every room, and the third one I checked had them, the other one was just as far from the start and has the biggest clue, I think what it said clicked for me until I did it. If that isn't clear I'll be clearer, I also tryed checking every tile in every room.