The loader should fit, but not sure if JD made 430 type mounts for the 3120. My 4300 tractor came with a 430, and the 300cx will also fit if i can find mounts. JD made proper mounts for the 300cx to the 4300. Finding them 20 years after the fact is another thing.
What is the year of the 430 you found. The early style bucket QD holders are a weak link, as are the claw style mounts. Late model 430 holders are cast like the 300cx but cost a bit more than the 300cx, but actually cost less than the early model inferior welded QD holders. I retrofit 300cx QD bucket holders onto my 430 loader.