Three of the four big broadcast networks now own stakes in Hulu, the popular video Web site.

ABC Enterprises, a unit of the Walt Disney Company, announced Thursday that it would join NBC Universal, which is owned by General Electric and Vivendi, and the News Corporation, owner of Fox, as a partner in the joint venture.

Hulu, which in the last 18 months has become the third most popular video site on the Web, behind YouTube and Fox Interactive Media, displays free, high-quality versions of television shows and movies, supported by advertising. It said it would add ABC shows like “Lost,” “Desperate Housewives” and “Jimmy Kimmel Live” to its online library by late summer, pending regulatory approval.

Anne Sweeney, president of the Disney-ABC Television Group, said that while most of the network’s shows will continue to be available on ABC.com, that site attracts mostly core fans. By distributing them on Hulu, Disney hopes to reach Hulu’s much-larger audience of 42 million visitors a month.

According to people briefed on the terms of the deal, ABC will give Hulu an exclusive license to distribute its shows on Hulu.com and across the Web on Hulu’s partner sites, like MySpace and AOL .com. ABC will also give Hulu around $25 million in marketing credit, which Hulu can use to advertise itself during ABC’s broadcast shows.