That is false information (as 300D would say

)
2TB is limit of APM, not PPC. AFAIK SATA controller doesn't have a drive size limit.
Over 2TB you need to use GPT. Any OS X up from 10.4.6 is able to partition, mount, read and write GUID. No matter if it's on PPC or Intel Mac. PPC won't only boot off GPT partition.
OP, you should be able to use 3TB HDD in G5 for all but booting purposes (i.e. storage, backup, scratch etc) in OS X 10.4.6 + or 10.5.