CONFIG_SUN3X is not available for thedefaultarchitecture x86.
Result is shown for architecture m68k

Sun3x support

configname: CONFIG_SUN3X

Linux Kernel Configuration
└─>Sun3x support
In linux kernel since version 2.6.12  
This option enables support for the Sun 3x series of workstations.
Be warned that this support is very experimental.
Note that Sun 3x kernels are not compatible with Sun 3 hardware.
General Linux information on the Sun 3x series (now discontinued)
is at <>.

If you don't want to compile a kernel for a Sun 3x, say N.