CONFIG_POWER3 is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

CONFIG_POWER3 (not configurable)

configname: CONFIG_POWER3

Linux/PowerPC Kernel Configuration
└─>Processor support
└─>POWER3
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
depends
CONFIG_PPC64