for(int i=0; i<project.Tables["mytable"].RowCount; i++){
    if ( project.Tables["mytable"].GetCell("A",i) == project.Variables["v1"].Value ){
        project.Tables["mytable"].SetCell("B",i,project.Variables["v2"].Value);
        project.Tables["mytable"].SetCell("С",i,project.Variables["v3"].Value);
    }
}