Explain at least three ways in which Plato demonstrates that he has drawn upon the ideas of Homer, the Pre-Socratics, Sophocles, and Euripides. Pay close attention to the participants in the dialog of the Symposium, the themes of their statements and the progression of their arguments. Base your conclusions on the works of Plato, and the other Greek authors.

In the Symposium, Plato has the banquet guests make numerous references to earlier Greek writers. Much is made, in one stretch of the dialogue, of Homer's alluding to Menelaus's having been an inferior to Agamemnon in battle but nevertheless coming uninvited to a feast. It is merely an incidental allusion, but a point is made about one of the interlocutors, Aristodemus, having come to the banquet uninvited, and it may have some bearing on the value of the comments Aristodemus makes within the dialogue. Eryximachus, another guest, then indicates he will make the exordium of his speech in the style of the Menalippe of Euripides. Eryximachus goes into a long quotation of a statement by Phaedrus, in which Phaedrus has lamented the lack of praise for Eros (Love, the main subject of the Symposium) "even by the sophist Prodicus" and others who had extolled the exploits of Heracles. Additional references are made to the "deeds of Love" by the gods as recorded by Hesiod and Parmenides.
These allusions to the Greek poets of the past, though they are done in passing, establish a framework in which Plato contextualizes the debate about Love. They also, to some extent, serve as a foil to the deeper thinking which is to follow. The centerpiece of the dialogue is, of course, Socrates's own analysis of Love, which he himself attributes to the foreign prophetess Diotima, Socrates being too modest to present the ideas as his own.
It is clear that Plato intends the speech of Socrates, which he withholds until the others have finished, to show not necessarily the wrongness, but at least the relative superficiality, of what the other guests have stated. Even if we as readers didn't know that Socrates was Plato's idol, this would be obvious from the fact that Socrates takes the discussion about Love specifically into a further realm where he talks about the motivations and drives that lie at the heart of human life overall. Other interpretations are possible, of course, and I am not entirely sure I fully understand all the points Plato has Socrates make. However, the Symposium incorporates a wealth of ideas, presented in the guise of a discussion among friends and rooted in the literature of Homeric and classical Greece.

The speech of Phaedrus in Plato's Symposium praises Eros, the god of love. In the speech, Phaedrus refers to many important earlier works by poets and philosophers in support of the importance of love and how love can inspire brave and noble actions.
An example of how love makes people brave and self-sacrificing, according to Phaedrus, is the case of Alcestis, the eponymous heroine of a play by Euripides, who volunteers to sacrifice her own life for that of her husband.
Phaedrus states that in Homer's Iliad, Achilles is inspired to return to the battle, despite knowing that it will lead to his death, out of his love and grief for his lover, Patroclus.
Finally, Phaedrus cites Parmenides as an example of an early philosopher who describes Eros as among the oldest of the gods.

In Plato's Symposium, seven very different men get together for a drinking party ("symposion" in Greek, literally 'drinking together') and talk about the definition and nature of love; each of the men offers an encomium to Eros, the god of love and desire. I'll mention three examples from the different speeches that show that Plato has drawn upon earlier thinkers:
1) In the speech of Erixymachus, the doctor, he makes a reference to the pre-Socratic natural philosopher Heraclitus when he talks about love, harmony, and the coming together of opposites. Erixymachus criticizes Heraclitus who thinks that opposites come together; Erixymachus argues that it is only similar things that are capable of being attracted, one to another. Aristophanes' tale of the round people is also presumably inspired by Empedocles.
2) Socrates refers to a story from Homer's Odyssey in his speech (at 198c):

For his speech so reminded me of Gorgias that I was exactly in the plight described by Homer: I feared that Agathon in his final phrases would confront me with the eloquent Gorgias' head, and by opposing his speech to mine would turn me thus dumbfounded into stone.

This is a reference to Book XI of the Odyssey.
3) At 199a, Socrates refers to Euripides' Hippolytus by quoting the famous line about his tongue having sworn but not his mind. This is but one of numerous references to Euripides. The very character of Agathon, the tragedian, alludes to Euripides and perhaps also to Sophocles.