Re: three computers, one external hard drive
First, and foremost, your computers all have to be on the same network. If they're all plugged into a router, you should be set.
Then, connect the drive to one computer, right-click on the drive, go to properties. The go to the sharing tab, and click the radio button to share the drive. You can set the specific name of the network share as well as click the boxes at the bottom if you want the other computers to be able to add or change files.
To connect the other computers, you need to know the name of the computer that the external drive is connected to. To find that, you can right-click on My Computer, go to properties, and go to the computer name tab. It's next to "Full Computer Name".
Now, on the other computers, to map the drive itself, go to My Computer--> Tools --> Map Network Drive. You can set the drive letter, and then in the Folder field, type "\\ComputerName\ShareName" without the quotes and where ComputerName is the name of the other computer that has the external drive attached, and ShareName is name of the share name that you set when you initially shared it. If this isn't working, you can also just type \\computername in the explorer bar, and it will come up with anything that computer is sharing. From there, right-click on the folder there and go to Map Network Drive. All the fields should be filled in, and you can select the drive letter. Click OK, and you're done. Rinse and repeat to connect the other computers.