VAYOMER MOSHE ICHLUHU HAYOM KI SHABBAT HAYOM LAHASHEM HAYOM LO TIM'TZ'U BASADEH.
The mitzvah to have three meals on Shabbat is Rabbinic (D'RABANAN) and it is linked to the more general mitzvah of ONEG SHABBAT, Shabbat enjoyment. There is also a link to the above quoted pasuk in the Torah (this week's sedra) concerning the manna and Moshe's instructions to the people for Shabbat - Eat (the manna) TODAY, for TODAY is G-d's Shabbat, TODAY you will not find it in the field. The three-fold use of the word HAYOM is considered a REMEZ to the three meals of Shabbat.
According to most sources and authorities, SHALOSH SEUDOT is a requirement for men, women, and children (one source says that women are exempt - this is included here, not to excuse women from this mitzvah, but just to be a bit more thorough in this presentation).
Preferably, Seuda Sh'lishit should also have LECHEM MISHNEH, the two loaves that are familiar from the two main Shabbat meals. This requirement seems to be a little lax for the third meal, with some sources not considering it as a requirement.
Based on some of the above, we can see an attitude problem surrounding Seuda Sh'lishit. Not its fault - ours. And that means we can do something about it.
First two Shabbat meals - women obligated with no other opinions. First two Shabbat meals - Lechem Mishneh, no second opinions. Kiddush precedes each of the first two Shabbat meals (with differences between them - but that's another story). No wine before Seuda Sh'lishit (recommendation in various sources to have wine during the Seuda Sh'lishit meal, but not Kiddush-like). Forget R'TZEI in Birkat HaMazon for the first two Shabbat meals - REPEAT the benching. Seuda Sh'lishit? No, don't repeat.
Erroneous conclusion in the minds of many people - Seuda Sh'lishit isn't so important. Wrong! It is. We (some of us) need a reality and attitude check.
Seuda Sh'lishit is one of the three required meals on Shabbat. Seuda Sh'lishit completes the set of Shabbat meals. Without it, you are just doing the weekday thing of two seudot. That's why Seuda Sh'lishit is also called Shalosh Seudot. Can't have three without a first and a second. Can't have three without the third one.
Being linked to the pasuk that started us off, means that Seuda Sh'lishit connects us to the manna. Moreover, it is like an extension of the manna. That less than 40 years period of a daily and weekly display of G-d's concern for His people carries on in our custom, practice, and ritual via LECHEM MISHNEH, via covering the Challot, and via Shalosh Seudot.
In halachic presentations of Seuda Sh'lishit, we find a reminder (more an imperative) to have in mind the Rabbinic mitzvah of three Shabbat meals. We are not just feeding our bodies - we are feeding our souls. Divrei Torah and Zemirot help in that respect. Seuda Sh'lishit refreshens our awareness and appreciation of Chasdei HaShem.
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