You can't do it using Format() function, becouse it use 24 hours format.
You need to write custom function, like this:
Function MyFormat(countOfSeconds As Long) As String
Dim h As Long, n As Long, s As Long
h = Fix(countOfSeconds / 3600)
n = Fix((countOfSeconds Mod 3600) / 60)
s = countOfSeconds - ((h * 3600) + (n * 60))
MyFormat = h & ":" & n & ":" & s & ":"
End Function