I get the following when attempting to read an x3f file using exif (but filing against libexif since I believe it to be a library issue rather than a frontend issue):
Corrupt data
The data provided does not follow the specification.
ExifLoader: The data supplied does not seem to contain EXIF data.
Here's an example x3f file:
http://img.photographyblog.com/reviews/sigma_sd15/sample_images/sigma_sd15_02.x3f
The exif front-end doesn't support x3f files. It should be able to decode the EXIF data within an embedded JPEG image if it were extracted by another tool.