This is one ugly macro, but it does get the right answer
Option Explicit
Sub test()
Dim E9 As String, E10 As String, E11 As String
Dim X As Long
E9 = "0:30"
E10 = "15:45"
E11 = "101:10"
X = 60 * (1 * Split(E9, ":")(0) + 1 * Split(E10, ":")(0) + 1 * Split(E11, ":")(0)) + (1 * Split(E9, ":")(1) + 1 * Split(E10, ":")(1) + 1 * Split(E11, ":")(1))
MsgBox CLng(X / 60) & ":" & (X - 60 * CLng(X / 60))
End Sub
Maybe a function to take "mm:ss" or "mmm:ss" strings and convert to integer seconds to calculating, and then format as "mmm:ss" to display