I think you are better off doing something like this:
Dim elapsedTimeList As New List(Of TimeSpan)
elapsedTimeList.Add(sw.Elapsed)
Then you can use Linq to get the largest and smallest
dim min as timespan = elapsedTimeList.Min();
dim max as timespan = elapsedTimeList.Max();
Then to format the timespan as a string do what you did before:
Example showing minimum:
String.Format("Minutes :{1}{0}Seconds :{2}{0}Mili seconds :{3}", vbLf, min.Minutes, min.Seconds, min.TotalMilliseconds)