CONFIG_AT91_DEBUG_LL_DBGU3 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Kernel low-level debugging on sama5d2

configname: CONFIG_AT91_DEBUG_LL_DBGU3

Kernel low-level debugging port
└─>Kernel low-level debugging on sama5d2
In linux kernel since version 4.3 (release Date: 2015-11-01)