3gs had multiple problems. Pics fuzzy, faulty earphone jack, screen scratched. Had wife's old 3gs with cracked screen, so I just ordered the front panel kit.
I used double sided tape on tablet paper, taking out the screws from each phone. I had multiple windows open to the Ifixit site, and switched a lot because I was trying to accomplish so many tasks. All went well until grandchildren came over for a sleep-over. Somehow that sticky tape got mushed up. So I bailed on the screen change over. I need to replace that one item, and will then go back and put on a new front panel. It was an awesomely wonderful experience, the tools high quality (the screwdriver is magnetized) and I now have a perfectly working 3gs!!!!
Have a ton of light, a headlamp, two pairs of reading glasses, lots of time, no children about, double sided clear tape, a tablet of paper that you can tear out, a pen, a laptop connected to Ifixit, a big table. I found that a lot of my 3gs problems (fuzzy pics and faulty speaker jack) were due to dirt inside camera. I vowed to wash my work pants daily to keep dirt out.