A Letter to Three Wives (1949)

Lora May Hollingsway, who grew up next to the wrong side of the tracks, married her boss who thinks she is just a gold digger. Rita Phipps makes as much money writing radio scripts at night as her school teacher husband does. Deborah Bishop looked great in a Navy uniform in WWII but fears she'll never be dressed just right for the Country Club set. These three wives are boarding a boat filled with children going on a picnic when a messenger on a bicycle hands them a letter addressed to all three from Addie who has just left town with one of their husbands. They won't know which one until that night.

Source: TMDb