"Home Box Office has been acting like they own cable TV, and a lot of us would like to prove to them that they don't," said one film executive of the recent offer three studios made to purchase 75 percent of the Movie Channel, the faltering Warner-Amex cable channel in competition with HBO. The problem, according to many top-level film executives, is that HBO -- which has 11 million subscribers compared with Showtime's three million and the Movie Channel's two million -- is becoming arrogant and tightfisted; says the executive, "they're basically saying, 'we're the only game in town, and you have to play by our rules.' " So MCA, Paramount and Warner Bros. have banded together to offer $100 million for three-fourths of the rival channel, in an effort to revitalize it and make it competitive with HBO. Shortly after that offer was made, its existence was leaked -- reportedly by HBO itself. If the studios succeed in buying the Movie Channel, it doesn't mean they'll be withholding any films from HBO, although that cable giant did find itself unexpectedly outbid by a consortium of subscription systems when it tried to secure the rights to an upcoming Frank Sinatra concert. But when Columbia, 20th Century-Fox, Paramount and MCA recently attempted to seal out HBO and license their films for exclusive nine-month periods to the Premiere pay-TV service, the Justice Department stepped in and stopped the action. This time, says the studio source, "we just want to make sure that we have more than HBO to deal with from now on."