First thing I want to point out is that you seem to be pre-assuming your position, and then trying to come up with arguments that support it, so that you can "win". This is, I believe, something we all often do, being more interested in finding arguments that make others think we're right than actually in being right. What I suggest is to first think by yourself, without standing in any position or having any bias, from a neutral and ignorant point, and coming up with arguments and counterarguments for yourself and all sides. Then, as you grow in wisdom in that matter, you may start having a position, which will feel more natural and according to what is true (or what is there of any truth).
I'm sure that if you do that you will come up with a lot of reasoning and arguments of yourself, maybe even change your mind a little, and in the process become more wise.
EDIT: So, applying this to your question, I would first start by trying to understand what is the theme of the discussion: Morality, that is, the rightness or wrongness of an action, and how should it be accessed. Of course, this brings up another question: what does it mean for an action to be right or wrong? Well, first, we have to know what an action is - it is something caused by an agent. Any kind of agent, like even the wind?, no, a self-conscious and in control agent (yes, this can give rise to a lot of discussion). This excludes any action that is not pre-meditated, like reacting impulsively, or stepping on an ant unaware. Now, this lets us understand that morality only applies to actions where there is some kind of free will able to impact the decision (and yes, this brings another lot of discussion, so you can see everything is linked). Okay, so now that we're a bit clearer on what is morality and what are actions, how can we tell if an action is right or wrong? Wait, why do we need/want to know how to tell if an action is right or wrong, why is it important to us? Well, as I see it that's because we have the notion of value, something which can't be described physically, the feeling or understanding that there are things which have an intrinsic, absolute value or importance, and want to act according to those values, or, in case of conflict, the highest value (I imagine this is the line of thinking through which Aristotle got to the same idea). The conclusion of this reasoning is then that we say an action is right or wrong whether it is or isn't according to a certain value; in morality, according to the highest or most important value (I wrote "certain" because we do not only use "right" and "wrong" with morality, but also when analyzing any action in relation to any value - example: "you are doing it wrong, that's not how we dance Gangnam Style", someone is not acting according to the value of dancing gangnam style properly). So we're entering here the line of thinking of Virtue Ethics. Now, some problems arise. What is then the highest value, what are the values we should follow? How do we know that a value is in fact the highest one? Of course, there's a question we need to ask first: What is a value? After some thinking, I've come up with the following: a value is anything of importance to a subject, and in accordance to which it acts; a moral value, however, is something which is intrinsically important, which is not important directly to the self but the subject values it because it sees in it an absolute, non-material importance. Now on to the question of what is the highest value, and how can we know so? For being such a subjective, hard to answer question, it gives rise to a lot of fear and consequent need for an answer. Thus, we often either look for an answer out there somewhere and take it as absolute or listen blindly (eheh) to what others say - and that's one possible explanation for what is called faith (the belief that someone or something other than us has the correct answer, in this case), and for many cases where people follow ideologies blindly (many times in religion, through which we tend to say that we've got the answers). The alternative is to try and investigate by ourselves the answer to those questions, and maybe that is the correct path because we, with our minds, are the only reliable source that we have for finding that out. Even if we can't yet reach for the ultimate answer, we are able to arrive to reasonable ideas and, with that, we create principles that we will to use to guide our actions the most accordingly possible to the highest value. In this way, we create imperatives - ideas or values that we should always follow in our actions to be the nearest possible to what is right. We share these imperatives with others, who may agree and follow them, or not. Both in this case, where we're following principles that we've come up or agree with, and when following ideas from outside, as when we say, for instance, that everything in a given religious book is the truth and depicts the highest values, we are following principles and believing that people should always act in accordance to a given imperative - it is our duty. That's how we get to deontology. Okay, so this is all very pretty, but what about this: what we want is to always act with the intention of promoting the highest value, or is it that our actions always be such that their consequences promote the highest value? Well obviously what we want is to impact reality and not just try to, so the answer is we want that the consequences of our actions always result in promoting the highest value, and not the scenario where we want to, but it doesn't happen. This is the main argument behind Consequentialism. Why don't we all follow consequentialism then? Well, in a perfect scenario where we are perfect beings and know everything and how everything will turn out and have complete control of everything, that ideal would be possible. However, we are not perfect beings, but human beings, and do not know everything neither control everything. Thus, I think we should focus on what we do control for now, meaning above all and absolutely our intentions, and work around that, while we try and evolve in two ways: try to discover each time more and get closer to the highest value, and try and become more wise so that we understand better our universe and get closer to the ideal situation of consequentialism, where we know how everything will turn out. Meanwhile, we can also enjoy the trip, and the ignorance we have and the unpredictableness of the universe.
I hope through this you agree with me that all these theories (Consequentialism, Deontology and Virtue Ethics) are not against each other but rather are variations around the theme of morality, being interconnected instead.
Taking your example of the murderer, I know I can't control all the consequences, even less when dealing with other human beings. So I would focus on what what I can control, which is my intention and actions. I don't want to kill, so I won't kill, and I can't tell if the murderer is actually going to kill the other two if I don't or change his mind, and I don't know if if I kill one he will keep to his word.
I hope I demonstrated how better it is to do what I suggested and think by ourselves, focusing on finding the truth.
I wrote my own thinking and it is probably full of mistakes and disagreements, and am hoping for the discussion on the comments :)