If its for display purposes, then select the field first. Then go to the 'Format' menu and select 'Date'.
If I change the field from "Format date as entered" to a custom date format in which I choose a short month format, I simply get a "?" display.
Are you doing this on the date field itself, not the calc correct. Again if this is for display purposes.
The custom date format should be set as the first choice using the 3 characters and the rest set as None.
Also make sure that the field is big enough to display it.
If not and you need to use the calc, then you can use a Left ( MonthName (YourDateField); 3 )
Yep, I needed to use the calc. Thanks!