I'd like to know if you can use the Korg D1 with a third party 3 Pedals Unit (or Third Party 2 Pedals unit)
No. You can use one pedal. One switch type sustain pedal is included by Korg and you have the option to separately purchase their half-damper capable pedal.
I'm going to buy my first piano, and I'm deciding between Roland FP-30, Korg D1 or Korg C1
Just based on hearsay, I'd vote for the Roland. (So, not much value in my opinion)
Some "flaws" have been pointed out on the cheaper Korgs such as low polyphony and noticeable velocity layer switching. We have talked about the D1 here before: http://forum.pianoworld.com/ubbthreads.php/topics/2742956/1.html
I'd like to know what would be the most similar model to the Roland FP-30 (compact design), but with the PHA-3 key action [...]
Roland doesn't use the PHA-3 anymore. It was replaced by the PHA-4 family of actions and that one has been partly replaced by the PHA-50. The last (and only?) portable model to use PHA-3 was the FP-80. (It was called "Ivory Feel-S" but it was part of the PHA-3 family.)
[...] since according to this post
, it is way better than PHA-4 Standard found on the Roland FP-30
That's only a personal opinion of a single person obsessing about the pivot length: "As the PHA-4 Standard has a "short" pivot length it must be worse."
It's shorter than PHA-3, but actually close to Kawai's plastic keys for example. And many people speak fondly of the PHA-4 Standard here whereas the old PHA-3 has been described as "noisy and hard bottoming".
[Photo from https://www.pianodreamers.com/roland-fp30-review/]
[Photo from a Kawai website]