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I recently installed it on a G2 so I guess it should also run on a G3. But I do think you would be better of using CentOS. Using CentOS you would be able to install the HP drivers for this server.
Besides official HP support matrix already mentioned, there is also http://wiki.debian.org/HP/ProLiant which might have some practical information. Not a very extensive list, but it shows that some G3 servers appear to be working just fine with Debian.
