CONFIG_ETRAX_SERIAL_PORT3 is not available for thedefaultarchitecture x86.
Result is shown for architecture cris
default or selected kernelversion does not have config value CONFIG_ETRAX_SERIAL_PORT3.
Result is shown for kernelversion 6.7-rc3

Serial port 3 enabled

configname: CONFIG_ETRAX_SERIAL_PORT3

Linux Kernel Configuration
└─>Serial port 3 enabled
In linux kernel since version 2.6.12  
Enables the ETRAX FS serial driver for ser3 (ttyS3).