The Asian online specialists are said to have tabled a bid worth more than $893 million to buy Valve, according to a "major Korean publication" (via Venturebeat).
The three companies reportedly met in Hawaii to discuss a deal, although both Nexon and NCsoft have refused to comment.
A Valve spokesperson did, however, dismiss the story as rumour, while Wedbush Securities analyst Michael Pachter believes that Valve is worth more than the reported bid.
Some have suggested that the US-based video game company is worth twice the $893 million stated in the original story.
The meeting between Valve and Nexon could have had something to do with their Counter Strike deal, in which Nexon serves as an Asian publishing and development partner.
NCsoft, meanwhile, has published MMO games such as City of Heroes, Aion and Guild Wars.
Most recently it published Guild Wars 2, which went on to top the all-format games chart in its first week on sale.
Electronic Arts has previously been linked with a buyout of Valve, although co-owner Gabe Newell is said to be reluctant to sell.
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