If you didn't understand much of the other answer, then fear no more.
Boot the workbench disk. When all has loaded you should see a disk which says Workbench3.0 and a disk which says Ram Disk.
If more icons turns up, it means that the harddisk is present. If this is the case you don't need the install disk, but if no other icons turns up, you need the install disk to partion your hd.
If a icon comes up, saying something like DH0, HD0, Workbench, Work or anything else, then try to doubleclick it. The disk should open and you can see everything on the disk.
You can check the name of your boot drive by press both mousebuttons at boot, and take a look at the Boot options menu. The drive with a priority saying something else than DF0 and CC0 is your HD boot drive....
To get a clean Workbench3.0 install, just do the following:
Press right Amiga and the letter e at the same time. Then a requester should come up. Type Cli and press enter.
If this doesn't work open the workbench3.0 disk, and find the Shell icon in one of the drawers, and double click the icon.
Type this as it is
format drive DH0: name Workbench ffs
Replace DH0: with the name of your boot drive. This will format you boot drive, and erase all files on the hd.
You can now copy files to the drive. To install the Workbench3.0 set, type the following
copy all df0: to Workbench:
This will copy all the files on the disk to the hd, also the files you can't see. Do this with all of the disk, and remove the disks from the drive.
Finally, reboot the computer, and if everything went well, Workbench will be loaded from your HD :-)