Yes, FreeBSD has a disklabel set for each slice/"MBR partition", while the disklabel of Net and OpenBSD are for the complete disk.
OpenBSD uses "c" to designate the whole disk, while for FreeBSD it symbolizes the slice.
NetBSD uses "c" or "d", depending on the hardware platform to designate the whole disk.
The disklabels of Net, Free and OpenBSD are thus not compatible.
I wouldn't advise to try writing from one BSD to another. You will be lucky enough if they can read each other
Development of the different BSDs at the filesystem and disk organization level has diverged too much.
If you put each BSD on a MBR partition/slice, you can format the fourth MBR partition with either FAT32 or ext2fs for data exchange. That, in theory, should give no issues.