سلام سلامت باشید ممنون 

 از این روش استفاده کنید : 

    public string GetCourseDuration()
        {
            int totalSeconds = 0;
            int totalMinutes = 0;
            int totalHours = 0;
           
                foreach (var item in Model.Episodes)
                {
                    totalSeconds += item.Duration.Seconds; //item.Duration = type: TimeSpan
                    totalMinutes += item.Duration.Minutes;
                    totalHours += item.Duration.Hours;
                }

                while (totalSeconds > 60)
                {
                    totalMinutes += 1;
                    totalSeconds -= 60;
                }
                while (totalMinutes >= 60)
                {
                    totalHours += 1;
                    totalMinutes -= 60;
                }
            
            return $"{totalHours:00} : {totalMinutes:00} : {totalSeconds:00}";
        }