I’ve been looking forward to mod my GOW3 for a while now and I got everything now but can someone please tell me which one works? One of them I saw on YouTube doesn’t have a working link and other people seem to be having problems with them… 
Well, I highly recommend Philymaster’s Gears of War 3 - GPD Editor since I used it myself, but, thanks to others like you trying to access the program and modify the game, the download link seems have expired. We’re all unsure if the link will ever be fixed and so, with that being said, I’ve read of others getting the program from other websites, but I personally don’t know where those people acquired it from so I guess you’ll have to try Google.
Best of luck!
P.S.
If you have any questions or concerns pertaining to the editor, if you do find it, then I highly recommend reading the official thread regarding the editor; rather then creating a brand new thread and asking the same questions. [CLICK HERE]