Try below code.
DateTime currentDate = System.DateTime.Now;
StringBuilder previousThreeDates = new StringBuilder();
DateTime previousDate;
for (int i = 1; i < 4; i++)
{
previousDate = currentDate.AddDays(-i);
previousThreeDates.Append(previousDate.ToString("dd/MM/yyyy"));
if (i < 3)
previousThreeDates.Append(",");
}
Response.Write(previousThreeDates.ToString());