CONFIG_ARCH_MX3 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

MX3-based

configname: CONFIG_ARCH_MX3

Freescale CPU family:
└─>MX3-based
In linux kernel since version 2.6.23 (release Date: 2007-10-09)  
This enables support for systems based on the Freescale i.MX3 family