I hadn't heard that there were three answers. Ten minutes of research seems to indicate that they can be characterized as: 'act for best consequences', 'follow rules', and 'be virtuous of character'?
'Be virtuous of character' seems vacuous. How are you supposed to decide what virtue is?
'Follow rules' seems at best silly and at worst evil. If you've made up your own list of rules, then again, you need some way of working out what's on the list. If you're following someone else's rules, then they had the same problem, plus you've now got to worry that they might be trying to get you to act in their interests, plus their rules might have been corrupted in the process of being transmitted from their head to yours.
Which leaves only 'act for best consequences', but of course, we need to say who is to judge the best consequences. If the judger is me, then surely that's the definition of evil? If the judger is some sort of average of everyone, then it defines a sort of altruism. I don't like either of those.
My personal morality has always been 'Do what you like'. It doesn't seem to have had (that many) bad consequences. I think that most people who know me think that I am a moral man. My main character defect seems to be that I tend to dislike people who bore me or whom I find physically unattractive, even if they are otherwise good people.
It seems unlikely that I have come to a better conclusion than 3000 years of accumulated philosophers. But then, if they've come up with any sensible answers, why are there three schools? Surely the correct ones should be able to convince the others that they are wrong. If they are arguing about anything real.
So further thoughts:
I can't even begin to work out what 'be virtuous' might imply, absent a definition of virtue. So I'll just ignore that one.
As far as rule-following goes, then, for instance, the old seem to be often quite keen on 'respect the old' as a rule. I'm old, and I don't think that I deserve any more respect than I did when I was young. I am more skilled and more knowledgeable. I don't need to tell you to respect me for that. I will be better than you at some things. That will make you respect me. On the other hand, I will be less mentally flexible than I was, and less physically strong, which will make me lose at some games that I would once have won. Why should you respect me for that? Pity perhaps. Make allowances for, perhaps. Tolerate, perhaps. But respect?
My advice to you, if you are young, is to respect people for and only for what they do, not for who they are. Recognize, however, that if you are twenty, then you only have a few years of experience of life as an adult, and so you are probably wrong about everything. Still. Lots of old people are also completely wrong about all sorts of things. So pick the models to follow carefully.
That might be an answer. Pick the old people that you want to be like, and find out what their rules are, and do that. But that's not really ethics. That's self-interest.
So screw following other people's rules as an ethical system. And how do I work out my own rules? And if I do, and then find that my rules end up making me do something that I don't want to do, should I change the rules, or do the bad thing?
And utilitarianism seems to be at least a system you could think about, but:
I am not an altruist. You can tell that because I am not starvation poor. If I were an altruist I would spend everything I own on helping the less happy. The charity Smile Train springs to mind. For £50 they claim that they can fix hideous deformities and so permanently and uncontroversially change people's lives immeasurably for the better. I will happily spend £50 on lunch. Which proves that I care more about lunch than I do about making stranger's lives incomparably better.
I am not claiming that working very hard and giving all the money to Smile Train is the best plan for a sincere utilitarian, but it is a plan, and whatever plan they are following must at least be judged against it.
So if you ever meet someone who claims to be a utilitarian, and yet they are not sleeping in a skip, ask them why.
If their answer is that their weighting of utility functions is biased heavily towards their own utility rather than to that of other people, then they are basically following my scheme.
Anyway, it looks as though there are three schools of thought, and two of them are silly, and one of them is not silly, but leads to the wrong conclusions, unless you calibrate the argument very carefully so that it leads to the right conclusions. Which makes it vacuous. Because you still need a way of working out which right conclusions you want to come out.
Is it possible that three thousand years of philosophy has been directed to deriving by pure thought reasons to do what we feel like doing anyway? If the conclusions are, as a result, roughly the same, but the derivations are all a bit silly, that explains why they are still arguing. But then why are there only three?
Does 'do what you like' have a posh greek name? Is it a sub-school of one of the major three? Why is it not the same as 'be evil'? What is evil if it is not acting in your own interests?