CONFIG_3_LEVEL_PGTABLES is not available for thedefaultarchitecture x86.
Result is shown for architecture um
default or selected kernelversion does not have config value CONFIG_3_LEVEL_PGTABLES.
Result is shown for kernelversion 6.2.5

Three-level pagetables (EXPERIMENTAL)


User Mode Linux/$SUBARCH $KERNELVERSION Kernel Configuration
└─>UML-specific options
└─>Three-level pagetables (EXPERIMENTAL)
In linux kernel since version 2.6.12  
Three-level pagetables will let UML have more than 4G of physical
memory. All the memory that can't be mapped directly will be treated
as high memory.

However, this it experimental on 32-bit architectures, so if unsure say
N (on x86-64 it's automatically enabled, instead, as it's safe there).