Ext3 isn't supported, FAT32 doesn't work for some reason either. I'm not sure how well it was tested before being added to FreeNAS because I haven't ever seen FAT32 (MSDOSFS) work. It doesn't seem to be a priority to fix them right now either.
I looked into this briefly and it looks like FreeBSD doesn't support Ext3 so it's not a FreeNAS issue. I also read that you could mount an ext3 FS as ext2 but from what I've read it didn't seem like it worked in all cases and you could almost be certain it will fail and corrupt your files if you have valuable data on the drive.