CONFIG_MOMENCO_OCELOT_3 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips
default or selected kernelversion does not have config value CONFIG_MOMENCO_OCELOT_3.
Result is shown for kernelversion 5.4.123

Support for Momentum Ocelot-3 board

modulename: fixup-ocelot3.ko

configname: CONFIG_MOMENCO_OCELOT_3

Linux/MIPS Kernel Configuration
└─>Machine selection
└─>Support for Momentum Ocelot-3 board
In linux kernel since version 2.6.12  
The Ocelot-3 is based off Discovery III System Controller and
PMC-Sierra Rm79000 core.

source code:
arch/mips/pci/fixup-ocelot3.c

is selected by
CONFIG_MV643XX_ETH