PRO TIP: ad a little bit of flux to all 3 pads. Then ad some solder to your soldering iron tip and build up the solder on the 3 contacts. Be careful to not damage the flex cable/ board by letting your soldering iron stay on the pads for too long (2-3 seconds each). My iron temp is set at 350F. Once that is complete, set your hot air gun on 350F and speed about half. Slowly wedge a spudger between the board and the battery flex. DO NOT PRY. You will damage the pads. Just let it sit there and hang out while you apply hot air, the soldier you added will liquify and the battery flex cable will pull itself up. When that occurs, you can remove the battery connector and replace with a new one. Make sure to ad a bit of flux to the pads on the board for attachment. For re-attachment: line up the 3 pads and use your soldering iron to put them down. Gently press down on the gold pad of the flex cable, and you will see the solder from the board liquify and come though the 2 holes. Repeat for the next 2 and YOUR DONE!
1: Its important when repairing any device that you buy quality parts from a reputable source. Hopefully most of you know now that saving $10-15 on a inferior part does not really save you money. It in fact, takes you longer to do and more frustration. BUY GOOD PARTS, PAY EXTRA FOR GOOD PARTS. THIS WILL SOLVE MOST PEOPLE'S PROBLEM. PERIOD.
2: When replacing the screen on LG G3 it's very important that the metal mesh, On the flex cable at the bottom of the screen... is placed EXACTLY as it was placed originally in the phone before the screen repair. VERY IMPORTANT(REFER TO STEP 10). If not the screen will go in and out. If you continue to have issues with the screen afterwards.. Try putting some CapTon Tape over it. That should resolve your issues.
PS: If that does not fix it.. its the screen or the device itself. I would suggest looking for any signs of water damage or small black marks on the Mainboard (implying a short) and take pictures to show your customer so they believe you.