I see the option for specifying three files (left, right, and ancestor), but it doesn't seem to be possible to actually display the ancestor.
The bottom pane shows the result of the merge. I'd rather see what the original content was, so I could understand the context for the left and right sides of the conflict. Is this possible? FileMerge has awful help documentation.
To my knowledge, this is not possible with filemerge.
Paid (but very good) app: http://www.deltopia.com/
Open source: http://sourcegear.com/diffmerge/