The correct procedure to install OS3.5 (and OS3.9 also) is as follows:
1. get your CD-ROM drive running on your old OS (3.0 in your case)
2. insert the CD, start the OS3.9 installer and create an emergency disk.
3. make sure the emergency disk works. It mainly contains the CD-ROM driver and then boots from CD, so it shouldn't be too difficult to check.
4. boot from the emergency disk.
5. format your former boot partition (with 3.0 on it)
6. install 3.1 on the now empty partition
7. install 3.5 pre
8. reboot
9. install 3.5 main
If you still get error messages, tell me the text so I can decide what it means.
If OS3.5 does not run on your machine, you should not bother getting 3.9, it won't run either. There is not much difference between the two (except the additional programs included with 3.9 and the ability to install 3.9 directly without 3.1 and pre installation before).
Bye,
Thomas