Right click on Cell B1 and choose Format Cells. In Custom, put the following in the text box labeled Type:
[h]:mm:ss.000
To set this in code, you can do something like:
Range("A1").NumberFormat = "[h]:mm:ss.000"
That should give you what you’re looking for.
NOTE: Specially formatted fields often require that the column width be wide enough for the entire contents of the formatted text. Otherwise, the text will display as ######.