CONFIG_ARCH_MX3 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
default or selected kernelversion does not have config value CONFIG_ARCH_MX3.
Result is shown for kernelversion 6.2.5

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

is selected by
CONFIG_MACH_VPR200