jeanmariecalvat, on the bottom of connector 3, there is a small black plastic tab that flips up 90 degrees with very light pressure. This will allow you to slide the flex cable in snugly. Once inserted all the way in (none of the copper contacts should be showing), then push the plastic flap back down snugly onto the pcb. It should work right now, unless you have damaged the connector itself.
Connector #3 is supposed to slide in from the top after you've flipped the tab up. Patience is key because its not as smooth as you'd like things to go! Ive found it helpful to guide the connector through the slot from the top, and then gently but firmly press down on the connector with a pen or a set of tweezers as you slide it through, and then flip the white tab down. If you are like me be prepared to use a good magnifying lamp, a steady hand and lots of patience. Oh yes, I noticed that some of the iPhone have the colors reversed. so if I have a black tab and you have a white tab, that is ok. Just take a good look at the connector.
Of course if you are in the business of some really, really small stuff soldering, I believe that here is the part you are looking for http://search.digikey.com/scripts/DkSear... and http://www.acos-electronics.com/sdp/5973... instructions are here http://forums.macrumors.com/showthread.p... I wish you good luck and please let us know how it is going.