Does fit a older version on a cd? (For example Solaris 8?) I've try to download it, but I can't find a download link. (Have I searched with my eyes closed? )
There are other ways to install Solaris 10 or newer without a DVD reader but you should first make sure your hardware has enough resources, especially RAM.
That's probably quite ambitious by today's standards. However, you might be able to run an X server and a lightweight window manager at a reasonable speed with some tuning.
Milax definitely supports SPARC along with x86 and x64 architectures.
Not without at least a Sun support contract, EBay is your best bet.
An Ultra-3 mobile has 550MHz or 650MHz processor, ok.
An Ultra-30 has a 250MHz or 300MHz processor, not up to the job.
Cannot find mention of an Ultra-3D?
I assumed that was a typo, there was indeed never a Ultra-3D. A 250 Mhz CPU can run an X server with no particular problem. A 50 Mhz SS10 did it flawlessly some time ago. The U30 graphic card support might however be an issue with Milax.
Finding a Solaris 8 media seems to be the most reasonable approach.
A friend said that it is a Ultra Sparc III but (I see it now) it's is maybe an Sparc Ultra III. (I feel me so stupid...... :()
I maked a picture of the front from my computer.
Solaris 8 to 10 are the most suitable OS to run but you might try several Gnu/Linux distributions including Debian PortsSparc - Debian Wiki and Ubuntu. Alternatively, your hardware is also supported by OpenBSD: OpenBSD/sparc64