After they've walked the carpet, how else should guests be greeted but with a flute of champagne or sparkling water? Either have trays of bubbly placed just inside the venue so they can pick them up as they enter, or have caterers personally serve the champagne glasses on shiny trays. You can also rent a champagne fountain so guests can help themselves.
If this is an Oscar party, you have three hours to fill. Have caterers serve easy-to-handle hors d'oeuvres at the beginning of the party as guests mingle. For dinner, you can make it easy on yourself by hiring a catered buffet.