There could be a couple of different format settings to allow this depending on your version of Android. A lower case 'l' (L) seems to do the trick for me. I have also seen specifications for 'sss' and 'fff' but those did not appear to work for me. So something like
"hh:mm:ss.l" as a DefaultValue should give you the milliseconds in TracerPlus.
Let me know if that helps.