To begin, it's worth noting that there is a somewhat quasi-mythical quality in play within all three of these traditions. The living person of Socrates has largely disappeared within the popular image of Socrates, as the philosopher questioning people in the marketplace, and later as a martyr to philosophy. Likewise,...
To begin, it's worth noting that there is a somewhat quasi-mythical quality in play within all three of these traditions. The living person of Socrates has largely disappeared within the popular image of Socrates, as the philosopher questioning people in the marketplace, and later as a martyr to philosophy. Likewise, the Buddha and Confucius exist at a similar distance, where what they have inspired (as well as the myths and stories created around them) have eclipsed the historical reality of their lives.
In terms of similarities, I think earlier contributors were right when they stressed the importance of duties and obligations (I'd suggest that ethics, spoken more broadly, was key to all three traditions). Additionally, it's worth noting that life in moderation (while avoiding the excesses of asceticism on one side and self indulgence on the other) are key themes in all three traditions.
However, even if we hold that ethics are a key component to all traditions, distinctions can be made. Confucius, for example, was largely focused on social interactions and the obligations and duties which people have with each other. His was a very unequal vision which tended to distinguish between a superior and an inferior, with each side of that relationship having duties and obligations to the other. Plato, by contrast, was first and foremost interested in the cultivation of virtues. Plato was a moral realist, who held that morality and goodness were written into the universe, as part of a higher transcendent reality (this is most famously expressed with his Theory of the Forms). While relationships, proper behavior, obligations, and duties might be part of this, ultimately the true goal which superseded all others, as far as a Platonist would be concerned, is to cultivate virtue in oneself. The same would apply to Aristotle.
Additionally, it should be noted that these different philosophies attempted to address different questions. For example, Buddhism is ultimately grounded in the problem of suffering, which is perceived as endemic within the human condition. If there is a core question within Buddhism, that question is this: why do people suffer and how can we overcome it? Confucianism, on the other hand, is largely focused on questions of social stability, so the problem of suffering does not have quite the same primacy. The same applies to the Platonic and Aristotelian traditions, with tend to place a greater focus on individual excellence and self-actualization.