With the current generation of cards, the Power Target is an "as needed" setting, rather than an instant voltage change.  In other words, the cards will use the amount of voltage necessary up to what you set it at.  If it needs less than what you've set them to, then they'll use less.  The first card is generally worked the hardest, so it's not surprising it would use the most voltage.  After you start OC'ing, you should see those voltages increase until you hit your stability wall or a natural voltage difference due to the quality of the silicon between the chips.  At this time, the cards appear to be operating normally.

Use GPU-Z to check the ASIC score of your cards.  If you right-click on the top bar, it will popup window and will have option to check what the ASIC score is.  My prediction is that #1 is the lowest score (60-70%s), #2 is higher (maybe low-mid 70%s), and #3 is the highest (probably in the upper 70%s or low 80%s).  If you don't know what ASIC score is, here is a good forum link that explains better. 
http://forums.anandtech.com/archive/index.php/t-2246288.html
 
It is not an exact science though.  But what it essentially measures is the "quality" of the actual GPU die.  Higher score, means higher "quality" chip.  This translates into lower voltages for the same or higher clock speed.  Conversely, lower score, lower "quality" chip, requires more voltage to achieve the same clock speed.
 
So there is nothing technically wrong with your cards.  It is just the difference in "quality" of the chip.  I am fairly confident that is what you are seeing.  If you want to overclock more, you will need to either get best you can based on the 2 cards with the higher voltages and have the one with lower voltage not be overclocked as much.  Or you could unlink them and overclock each separately.  This way you can tailor the overclock to each individual card.
 
Hope this helps.  Good luck!