CONFIG_DEBUG_BCM_IPROC_UART3 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Kernel low-level debugging on BCM IPROC UART3

configname: CONFIG_DEBUG_BCM_IPROC_UART3

Kernel low-level debugging port
└─>Kernel low-level debugging on BCM IPROC UART3
In linux kernel since version 4.2 (release Date: 2015-08-30)  
Say Y here if you want the debug print routines to direct
their output to the third serial port on these devices.