Barbara made landfall in the southern state of Chiapas on Wednesday, producing heavy rains that drenched the neighboring states of Oaxaca and Guerrero. The storm was downgraded to a tropical depression on Thursday.
The latest victim was a 60-year-old man whose body was found late Wednesday on a river bank in the Oaxaca town of Santa Cruz Zenzontepec, state police said.
The other victims were a 61-year-old US surfer who was slammed by the waves in Oaxaca's Playa Azul and a 27-year-old man who was dragged away by a swollen river.
The navy and local authorities meanwhile resumed their search for a dozen fishermen who have not returned since leaving on four boats Wednesday, said the mayor of San Pedro Tapanatepec, Ricardo Escobar.
Escobar voiced confidence the men would be found since two other fishermen who had been reported missing were located late Wednesday "alive and in good condition."