uTimerLib.h line 150 references _VARIANT_ARDUINO_STM32_ and this is for some reason not defined when I use Nucleo32 / NucleoG031K8, so the library compile borks because it doesn't know anything about Timer3.

As a workaround I created a platform.local.txt file with a single line

compiler.extra_flags=-mcpu={build.mcu} {build.flags.fp} -DUSE_FULL_LL_DRIVER -D_VARIANT_ARDUINO_STM32_ -mthumb "@{build.opt.path}"

To clean this up I would suggest to use ARDUINO_ARCH_STM32 instead of _VARIANT_ARDUINO_STM32_.