Most of it was pretty easy, but getting the pad_zeros to work was a bit hard. But I was pretty excited when I thought of int2str (I thought there was something like that but didn't find it) and started to look in the documentation. Anyway, this is my result:
func update_interface() -> void:
var coins_collected: String = String(PlayerData.score)
score_label.text = "%s" % coins_collected.pad_zeros(3)
But now I want to change it so that the PlayerData caches the coins collected when going to the next level so when you die, you don't start at 0 and you can't keep dying to collect infinite coins.
Wish me luck XD