able to mount ad0s1, but unable to mount ad0s3!

[Kenny]

Well, my FreeBSD reside in ad0s2. While  ad0s1 and ad0s3 is for Windows.

Both ad0s1 and ad0s3 are NTFS. I can easily mount ad0s1 by typing the
following command:
mount -t ntfs /dev/ad0s1 /mnt

However, when I tried to mount ad0s3 using the same command, it does
never work. The error message is just as this:
Kenny# mount -t ntfs /dev/ad0s3/ /mnt
ntfs: /dev/ad0s3: Invalid argument


It seemed no use for me to get help from the handbook. What is wrong?
Ohhhhh, I am quite puzzled.
Waiting for your help, thanks!