OK, I haven't read the 3rd edition yet, but one of my best friends has bought the novel and told me about the changes Jin Yong made. My friend is a bit of a Jin Yong purist, so I completely trust the info he has given me.
In the new edition, Zhang Wuji learns about Nine Yang's origins on the last page of the Lankavatara Sutra. It states that Wang Chongyang, after having obtained the Book of Nine Yin, travelled to Songshan one day and had a drinking contest with a certain guy. Wang Chongyang lost, and had to let the man read the entire Book of Nine Yin as a favour. This man used to be a Confucianist scholar in his youth and was exceptionally bright and educated. He later became a Taoist priest and acquired a vast amount of knowledge about Taoist principles and theories. But when he reached old age, he became a Buddhist monk for some unknown reason. When reading the Book of Nine Yin, the monk, with his superior intellect and knowledge, noticed that the Nine Yin martial arts were based on extreme Yin principles instead of a harmony between Yin and Yang. Realising that there must be "another way", he created the Art of Nine Yang and wrote the Book of Nine Yang inside the Sutra. This sutra somehow got into the Shaolin temple, so maybe the monk was a Shaolin member.
Now, there are a few things about this whole matter that I find illogical/strange:
1. Wang Chongyang obtained the Book of Nine Yin when he was already quite old. Although Wang Chongyang was a valiant, heroic warrior in his youth, it seems odd that the elderly, wise Wang Chongyang would have a drinking contest, let alone put the Book of Nine Yin on table as a stake.
2. The other contestant used to be a Confucianist scholar, then a Taoist priest, and later still a Buddhist monk... I am sure such persons did exist in Chinese history, but I guess they were few, and Wang Chongyang just happens to meet such a one? Clearly Jin Yong needed someone to fill the vacancy of "gifted and learned, with good knowledge of Taoism and willing to write stuff inside a Buddhist sutra".
3. OK, this guy borrowed the Book of Nine Yin for a short period of time to read the whole thing. He must have a superb memory (similar to Huang Rong's mom) to be able to remember the contents. The supreme genius Huang Shang needed several decades to create the Art of Nine Yin. The martial arts prodigy Wang Chongyang read the book, pondered for more than 10 days and was only able to absorb the knowledge of the book, he could not surpass it nor could he add something new to it. Yet this monk was able to create a method which rivals Huang Shang's invention and surpass Wang Chongyang's knowledge! And he only read it for a while! Not impossible... but extremely unlikely. It's freaky that there are so many geniuses around during that era, and they all met and knew each other. Strange...
Anyway, I don't like it. My friend doesn't like it. Might have been better if the origins of Nine Yang remained a mystery, explaining everything sometimes spoils the fun.