Three Keys Of Shabbos
במדרש יפתח ה' לך כו' אוצרו הטוב ג' מפתחות בידו של הקב"ה ע"ש
In the Midrash on the pasuk "Hashem shall open for you... His good treasury" (Devarim 28:12), the Sages teach that there are three keys held in the hand of the Holy One, Blessed is He, as is explained there.
The Midrash on Hashem opening His good treasury teaches that the Holy One, Blessed is He, keeps three keys in His own hand and does not entrust them to any agent.
והם בני חיי מזוני שהם למעלה מהנהגת הטבע לכן לא בזכותא תלי'
These are children, life, and sustenance, which stand above the natural order of conduct, and therefore they do not depend upon merit.
These three keys are children, life, and sustenance. Because they transcend the system of nature, they are not granted according to a person's merits but flow directly from Hashem.
וזה מיוחד רק לבנ"י וע"ז כ' מה רב טובך אשר צפנת
This bond is unique to Bnei Yisrael alone, and concerning this it is written, "How abundant is Your goodness which You have stored away" (Tehillim 31:20).
This direct relationship belongs uniquely to Bnei Yisrael, and Dovid HaMelech alludes to it when he speaks of the hidden goodness Hashem has stored away for them.
ומזה יש התגלות בכל שבת כדכ' ביום השבת יפתח
From this there is a revelation on every Shabbos, as it is written, "On the day of Shabbos it shall be opened" (Yechezkel 46:1).
This hidden goodness becomes revealed every Shabbos, as the navi Yechezkel hints when he says the gate is opened on the day of Shabbos.
כי בחול ההנהגה ע"י מלאך
For during the weekdays the conduct of the world is administered by means of an angel,
On ordinary weekdays Hashem runs the world through an intermediary, namely an angel.
ובש"ק כביכול בידו של הקב"ה בעצמו
while on the holy Shabbos it is, as it were, in the hand of the Holy One, Blessed is He, Himself.
On Shabbos, however, the conduct of the world returns, so to speak, to the direct hand of the Holy One, Blessed is He, Himself.
ולכן הג' סעודות מיוחדים לאלה הג' בח"מ [ובליל ש"ק יתכן שהוא בחי' מזוני
Therefore the three Shabbos seudos correspond to these three matters of children, life, and sustenance. On the night of Shabbos it would seem to be the aspect of sustenance,
Since the three keys are revealed on Shabbos, the three seudos align with them: the Friday night seudah corresponds to sustenance.
וביום השבת חיי
on the day of Shabbos it is the aspect of life,
The Shabbos day seudah corresponds to the key of life.
ובסעודה ג' בני כפי מ"ש בתיקונים בכמה מקומות ע"ש ד' ס"ז סע"ב
and at the third seudah it is the aspect of children, in accordance with what is written in the Tikkunim in several places, as is explained there, daf 67, second side.
The third seudah, seudah shlishis, corresponds to the key of children, as the Tikkunei Zohar teaches in several places.
ולכן בליל שבת בעי למיטעום מכלא דבי' שרי' ברכה במזוכי כנ"ל] ומיני' מתברכין ימי המעשה ג"כ כנ"ל:
And therefore on the night of Shabbos one must taste from everything, since it is from there that blessing rests, in the place that is refined, as above. From there the weekday days of labor are likewise blessed, as explained above.
Therefore at the Friday night seudah one tastes from every dish, since blessing settles upon that refined source, and from Shabbos that blessing then flows out to bless all the weekdays of labor.
Summary: The Sfas Emes builds on the Midrash that the Holy One, Blessed is He, holds three keys in His own hand: children, life, and sustenance. Because these stand above the natural order, they are not distributed according to merit but flow from Hashem directly, a bond unique to Bnei Yisrael, as alluded to in the goodness He has stored away. This hidden treasury is revealed each Shabbos, when the world is governed by Hashem Himself rather than through an angel as on weekdays. Accordingly, the three Shabbos seudos correspond to these three keys: sustenance at the Friday night seudah, life at the day seudah, and children at seudah shlishis, as taught in the Tikkunim. One therefore tastes from everything at the Friday night seudah, where blessing rests upon the refined source, and from Shabbos that blessing flows forth to sanctify all the weekdays of labor.