Do you even have a diskette named "Install 3.0"?
The A1200s that were sold without hard drive also didn't come with "Instal 3.0" diskette.
Do you even need an install disk?
The last time I installed workbench I booted the floppy, formatted the hard disk, then simply copied the contents of the workbench copy onto the hard disk and (I think) moutned the drive.
Hey presto, one bootable hard disk. No need for install disk.