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In Kant's categorical imperative, he outlines 3 maxims:
Universal moral law: ‘Only act on a maxim that you could will should become a universal law.’
Treat people as ends: ‘Act so as to treat others and yourself always as ends, never simply means to ends.’
The kingdom of ends: ‘Act as if through your maxims you were a law-making member of a kingdom of ends.’
Isn't the third formulation just a combination of maxims one and two? If not could you offer further explanation?
I think this is a slight misunderstanding. While the Formula of Autonomy (Ak. 434) is in some sense combining the Formula of Natural Law and the Formula of Humanity (see Ak. 436), the "kingdom" of ends (translation problem! "Reich" is defined as "systematic interconnection of rational beings through their laws" (own translation, Ak. 433), kingdom is misleading here!) adds an interpersonal component that reaches further and is thought as a limitational concept [Grenzbegriff] to the kindom of nature [Reich der Natur] in which it should be thought as realizable (Ak. 438).
Regarding the actual formula: The difference is, as already pointed out, the systematic interconnection by laws. You may be able to abstract from your subjective standpoint regarding one maxim, thinking it as a law of nature. The same with one maxim considering the end-in-itselfessness of rational beings. But it is another thing to think all this within a system of such laws and individuals. It highlights in some sense the consequences of my recognition as lawmaking in other rational beings.
BUT in Ak. 436 it is clearly stated that "these three formulas" are "objectivly-practical" the same. While it is still heavily discussed which three formulations are meant here, it seems clear to me that they all only do highlight one aspect of the concept of the categorical imperative each, but do not loose their essential identity as types of it (the formulation from the later Critique of Practical Reason). So technically speaking, they are all the same and combining them does not add anything.
All formulations represent only different ways we can grasp the categorical imperative with intuitions [Anschauungen], by types (this is already implied by the "subjective-practical difference" in the GMM, Ak. 436), as there cannot possibly be any intuition of it as such, because we are not capable of pure intuitions, i.e. we cannot directly apply it in the realm of nature (thought as the sum of all experience) because it does not obey our boundaries of experience (the categories). We have no scheme (Schema) for it in its "perfect", general form. These points are also from the second Critique.
Last point, restating and fleshing out the second sentence of this answer after all: The universal moral law is the general rule including all these aspects simply because of its generality, leaving out any specification (Ak. 436-7 and 402/420-1; hence the Foundational Law of Morals in his second Critique), therefore the aspects thought as combined in GMM are the one highlighted in the Formula of Natural Law with the one in the Formula of Humanity (under above mentioned restrictions).
Henry E. Allison discusses this topic to some extent in his Kant's Groundwork for the Metaphysics of Morals: A Commentary, but gets to conclusions different from mine in some points. It would nevertheless be a good starting point for further readings.
Each form is equivalent to either of the other two from a different perspective, so there is no combining, they are just all the same.
To put it very elliptically:
Universal interchangeability of perspectives relative to every moral law implies we must all have autonomy (or no one would). So form 1 implies form 2.
Autonomy comes with agency, including responsibility to systematically protect everyones autonomy. So form 2 implies form 3.
And protecting everyone's autonomy implies you should not treat people too differently, so moral laws should be univeral. So form 3 implies form 1.
