here are three opinions given for the proper arrangement of the seder table. (I have not found the source for the circular arrangement often portrayed on specially made plates.)
The Ar"i za"l's seder plate is the most common custom. It is based on two triangular patterns, two images of the segol vowel mark. This is to invoke the notion that the Jewish people are the "Am segulah", the treasured nation. Also, the three matzos, the seven items on the ke'arah, and either the cup of wine or the plate itself, make ten items. The Ar"i za"l's arrangement shows them in correspondence to the 10 sephiros of the Tree of Life.
The Gr"a's seder table stresses the memorial aspect of the beitzah (egg) and the z'roa (shank-bone). For this reason they are placed up front, so that you must pass them and think about them to perform any of the mitzvos of the seder.
The Ram"a bases his arrangement on the halachic principle of "ein ma'avirin al hamitzvos", one may not pass by a mitzvah. Therefore the items used first are closest, and those not used at all are in the back. (Aspaqlaria)
It is a custom in most of Israel to have, on the Seder table, three matzos under a cover. Why three? In the seuda (festive meal) we make "HaMotsi" over the matzos, and at any Shabbos or YomTov we need a second challa ("lechem mishne") when we make "HaMotsi" at the seuda. But why do we then need three matzos instead of two? Well, in the course of the seder, at "yachats" the middle matza is broken in two, and half removed (the "afikomen"). If we only had two matzos to begin with, we would end with one and a half, which is inadequate for the lechem mishne. Therefore we start with three. This is the opinion of Rashi, Tosfos, and the Rash, and it is the ruling in the Shulchan Aruch.
However not all commentators agree with this opinion. The Gemora, in Pesachim (115A), only says that one of the matzos must be broken (not necessarily the "middle one"), and the Rambam (Maimonides) and the Gr"a (Vilna Gaon) were of the opinion that there should only be two matzos. In fact, the Rambam thought it was incongruous that there should be two loaves of bread at Shabbos and all the other festive seudos, but a larger number (three) on Pesach, when the matza is supposed to be "lechem oni", the "bread of affliction"! Today the followers of the Gra in Jerusalem use only two matzos. So did Rabbi Moshe Feinstein (zatzal), and so does Rabbi Joseph Soloveitchik (zatzal).
I recommend that in this matter you follow whatever your family custom is.
Now I'd like to turn to another halachic issue. What is the shiur of matza, that is, the amount required to be eaten to fulfil the mitzvah of eating matza? Rashi, Tosfos, the Rash and the Shulchan Aruch say it is two "kezeisim". (A "kezais" is the equivalent of an olive in Biblical times, about one ounce or 27 grams. A shiur of two kezeisim corresponds roughly to almost all of a typical round shmura matza.) Why two kezeisim? Well, we make two brachos when we eat the matzo: "HaMotzi lechem min ha-aretz" and "VeTzivanu al achilas matzos". So these commentators say we are doing two mitzvos here: the mitzvah of eating a seuda including bread, which we do at all festivals, and the special mitzvah of eating matza. Each mitzvah requires one kezais.
The Rambam and Gra disagree and require only one kezais. Now, I suggest that the reason for this is that they believe that there is only one mitzvah involved here. (Of the two brachos mentioned, only "al achilas matzos" would then be a "birkas ha-mitzvos", a bracha on a mitzvah. "HaMotsi" would be a "birkas ha-nehenin", a bracha on enjoyment.)
From this it would also follow that the two controversies are over the same issue! If (as Rashi and the Shulchan Aruch say) there are two brachas involved here, then it is only the mitzvah of eating matza that is associated with "lechem oni", and for the second mitzvah of eating a seuda including matza, the concept of "lechem oni" does not apply, so there is no need to restrict ourselves to two loaves. But if (as the Rambam and the Gra seem to imply) there is only one mitzvah involved here, then indeed the notion of "lechem oni" applies to the matza eaten for the seuda.
There remains the problem of justifying, according to the Rambam's and the Gra's understanding, the fact that there are only one and a half matzos for the bracha of "HaMotsi", instead of two complete ones according to the requirement of "lechem mishne", and I'd like to finish off by sneaking in a homiletic word to try and explain this. In Tehillim (Psalms) it says: "A broken heart is dear to G-d". And the Kotsker Rebbe has said: "A broken heart is the purest". This doe not mean that G-d wants our hearts broken, but that when there is affliction in our lives, we are stripped of all our pretensions, and so purer. How does this relate to Pesach? Bread, remember, is puffed up mainly with air, and many people, when you meet them in normal times, have a front, a facade, that consists mainly of air. But when times are hard, when there is "oni" or affliction, then the air, the puffery, goes out of people, and they are flattened, like matza, to a purer state. In the seder, at yachats, we take this concept of "oni" one step further, and break a matza in half. Normally, during the year, we need two complete challos at Shabbos or YomTov, and in fact if we use matzos for the challos, we must first check them carefully under the light to see that they are "lechem shalem", complete bread, without even any cracks. Now I suggest that at Pesach, at the Seder, this second matza is "shalem", complete, precisely because it is broken, and pure, like a broken heart. (R' Haber)
In addition to being a reminder of the Passover offering, the roasted egg is a symbol of the Jewish people. The longer other foods are cooked, the softer and more tender they become, but the longer an egg is boiled, the harder it gets. Similarly, the more painful and severe the hardships of the exile, the stronger and more resilient the Jewish people emerges. (Rabbi Yehudah Prero)
We can divide the seder into four sections, each part corresponding to what is done and said with each cup of wine before us.
The steps of the Seder are:
First cup: Slavery
1) Kadesh; 2) Ur'Chatz; 3) Karpas; 4) Yachatz;
Second cup: Telling of the transition from slavery to freedom
Fourth cup: Praising G-d from a position of freedom
14) Hallel; 15) Nirtzah.
The mishnah in Pesachim tells us that we are to “start with modesty, and end in praise”. We see this pattern not only in the phraseology of Maggid, but also here, in the seder as a whole. Each cup presents a theme. The first cup, followed by washing for karpas, dipping a vegetable in salt-water, and breaking a matza in two, much as a poor person would not want to eat all he has at once, all are symbols of slavery – modesty. The second cup is over Maggid, retelling the redemption, and the third is before us during the re-enactment of the redemption. Last we have hallel and nirtzah, positioning ourselves from a post-redemption viewpoint – the praise. (Aspaqlaria)