3rd LED means '''your video card is not communicating with the logic board'''. See [http://www.ifixit.com/Answers/View/76612/Where+can+I+find+the+diagnostic+LEDs+on+first+iMac+Intel#answer76654|this Answer]. The video card can not be replaced or repaired separately, you have to replace the entire logic board. ''IMHO, I would put the money towards a new computer rather than pay the $300+ to fix/replace the board.''
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