MacOS – External hard drive has 3 partitions, all 3 are about full and appear on desktop but finder shows no content

Before you start, make sure you have a verified working clone of both partitions/systems.

You will ideally want to have software that can clone partitions, such as either the excellent CarbonCopyCloner, or the equally awesome SuperDuper, both of which will allow for your needs (complete clone) in their Shareware mode. I don't know what tools are suitable for PC, but find one that does the equivalent for Windows installations.

Reboot from your cloned Mac OSX installation, and prepare to repartition your target Hard Drive. Disk Utility can sometimes repartition volumes non-destructively, but for this I would recommend quick erasing the drive and repartitioning to the desired sizes for each OS, with a caveat for the situation below.

Repartition as desired (ensure GUID, journaled, case-insensitive for the Mac Volume), noting you may need to go through various procedures if you are using OSX 10.7 Lion and wish to retain the hidden recovery partition (ie: you may have to reinstall Lion and restore from your backup, then proceed with Bootcamp installation which will again walk you through creating a suitable PC formatted boot volume which you can later clone your Windows installation back to).

Clone back the OSX Installation that you are booted from to the target Mac partition on the internal drive (if you haven't already restored from this in the previous step). Do the equivalent process in Windows for your Windows installation.

Test each by booting into each and making sure everything works as expected.

Obligatory warning: You seem to have found your way to fdisk. Keep in mind that, like most BSD-Unix-style command line fdisk tools, the Mac OS X fdisk's editing feature does not 'do' safeguards, it has even less safeguards than whatever other platform's command line fdisk tool you're thinking of. It will happily let you accidentally use the erase command (which removes all the partitions) or edit a partition that Mac OS X is currently using (rendering your system unbootable) and even has few safeguards against putting numbers that just plain don't make any sense under any circumstances right into the partition table, so please be careful. Its only saving grace is that the changes are only in memory until it writes them, so if you are careful you can exit without saving.

You can use fdisk in interactive mode to edit the entries in the partition table. For instance, to edit the partition table on disk 1:

sudo fdisk -e /dev/rdisk1

Then edit your first blank partition table entry (e.g. edit 3), set it to type AF, press return for the default to the CHS mode question, press return to accept the default of starting the partition after the end of the last one, then enter the end sector number you want (the default is the end of the disk; to figure out one for a size you want: divide the size you want by 512 bytes to get the number of sectors you want, and then add that to the start sector number and subtract one to get the end sector number ). Do a print to make sure it looks okay (nothing has been saved yet, so if there's a mistake you can just exit (not quit) at this point and run fdisk again to start from the beginning.) Then write to save the partition table over the old one on the disk. Then exit.

If you get a warning at write that the changes will require a reboot, that's okay; answer y, and reboot Mac OS X after exiting.

Now the partition table has been updated, but the partition itself still has the whatever old blocks of data were in that space before; Run Disk Utility, and from the Erase tab, choose the new partition, give it a name, and Erase it.