This is an interesting question (or cluster of related questions), because it highlights the complexity of understanding both what ethics is and how different approaches to ethics "work."
First off, there aren't really "three branches of ethics". There's three main ways that some people have categorized approaches to ethics -- but it's not clear these are the only three. Where, for instance, does care ethics fit into the picture? Also is moral relativism an ethical position? Or moral nihilism?
That being said, we can now to turn how you've suggested we define each of these approaches to ethics and your central question about whether each
presuppose[s] that you have something.
Let's start with the definition you supply for deontology:
Obey a list of moral rules
This is pretty run of the mill definition that some people might give for "deontology" but "deontology" is also supposed to describe Kant's ethics. And this definition would run up against several problems for Kant. Kant sees acting ethically as not merely obeying "moral rules" but importantly generating these rules through the autonomous use of reason and then willing to follow them (Groundwork sections 1 and 2).
Some contemporary spins on Kant (versions? interpretations?) like Christine Korsgaard understand the binding of moral law on the individual as a consequence of acting rationally. In other words, they specifically reject that this is an arbitrary presupposition. Instead to act and all while using "reason" is to commit oneself to following certain rules that are fundamentally rational.
Returning to the definition you suggested, it seems an amendment is necessary:
deontological theories are those where a person's action is right or wrong in relation to a rule (and that rule's justification may vary depending on the particular theory).
We can now turn to virtue theories, you suggest:
Virtue Ethics: have a list of virtues to cultivate
This definition is interesting in that it's half right. If we look at Aristotle's Nicomachean Ethics, what we find in Book I is that the justification for virtue theory is "the function argument" (I.8), which states that things are "best" when they excel at what they are. After that, Aristotle does give a list of virtues (arete -- meaning excellence) in Books III-VI or so, but the important point is that Aristotle is identifying morality with excellently performing the human function. So then at least for Aristotle, the list is not arbitrary -- it's a list that he thinks follows from human nature.
Other virtue theorists may have different ideas either about human excellence or how we should pick the virtues we should try to excel at (I'm not sure if objective list theorists are committed to the tight metaphysical connection between human nature and the things we should pursue -- or even if they view their position primarily in ethical terms).
So again we can see that the definition needs amendment especially if want to focus on justifying the list of virtues in a virtue theory:
a virtue theory is a theory where right action centers on cultivating some set of virtues (where the set of virtues depend on metaphysical considerations and arguments that depend on particular virtue theories).
If we turn to utilitarianism / consequentialism, then a similar issue will arise. What motivates any particular consequentialism to think that only consequences matter? Mill has an argument for why good/bad just mean pleasure and pain.
If I could answer your central question in a single sentence, it would be: it's not that these approaches do not provide justifications but that categorizing different theories into one of three boxes in this way does not highlight the justifications that each individual versions proposes.
In fact, most major views in ethics argue at length for the justification of why they think actions should be understood in their particular way. Thus, we have Kant attacking the utilitarians of his day. We have Aristotle arguing that pleasure/pain are only good indicators for a phronemos (rather than the common man). We have Mill arguing that at the end of they day all other theories boil down to pleasure/pain despite how they wrap it in other things...
- Kant, Immanuel. Groundwork of the Metaphysics of Morals
- Korsgaard, Christine. Self-Constitution
- Aristotle Nicomachean Ethics
- Mill, JS Utilitarianism