"And they went three days in the wilderness and found no water" (Exodus 15:22). Rabbi Yehoshua says: According to its plain sense. Rabbi Eliezer says: But was not the water beneath the feet of Israel, since the earth floats only upon the waters, as it is said, "To Him who spread out the earth above the waters" (Psalms 136:6)? Then what does Scripture teach by saying "they found no water"? Only that it was to weary them. Others say: The water that Israel had taken between the partitions of the Sea ran out for them at that hour. Then what does Scripture teach by saying "they found no water"? That even in their vessels they found none, like the matter of which it is said, "and their nobles sent their lads for water" and so forth (Jeremiah 14:3). The expounders of hidden meanings say: "They found no water" means words of Torah, which are compared to water, as it is said, "Ho, everyone who thirsts, come for water" (Isaiah 55:1). Because they had separated from words of Torah for three days, they therefore rebelled. For this reason the prophets and elders instituted that they should read from the Torah on the Sabbath, and on the second day, and on the fifth day. How so? They skip one day after the Sabbath and read on the second day; they skip the third and fourth days and read on the fifth; and they skip the sixth day. Originally they instituted three verses for one man, corresponding to priests, Levites, and Israelites. Ezra came and instituted three verses [per reader], corresponding to the ten men of leisure.
Three Days Without Water as Three Days Without Torah
Yalkut Shimoni on Torah 255:3
וַיֵּלְכוּ שְׁלֹשֶׁת יָמִים בַּמִּדְבָּר וְלֹא מָצְאוּ מָיִם. רַבִּי יְהוֹשֻׁעַ אוֹמֵר, כְּמַשְׁמָעוֹ. רַבִּי אֱלִיעֶזֶר אוֹמֵר, וַהֲלֹא הַמַּיִם תַּחַת רַגְלֵי יִשְׂרָאֵל הָיוּ וְהָאָרֶץ אֵינָהּ צָפָה אֶלָּא עַל הַמַּיִם, שֶׁנֶּאֱמַר (תהלים קלו, ו) "לְרֹקַע הָאָרֶץ עַל הַמָּיִם", הָא מַה תַּלְמוּד לוֹמַר לֹא מָצְאוּ מָיִם, אֶלָּא כְּדֵי לְיַגְּעָן. אֲחֵרִים אוֹמְרִים מַיִם שֶׁנָּטְלוּ יִשְׂרָאֵל בֵּין הַגְּזָרִים שָׁלְמוּ מֵהֶם בְּאוֹתָהּ שָׁעָה, הָא מַה תַּלְמוּד לוֹמַר וְלֹא מָצְאוּ מָיִם, אַף בִּכְלֵיהֶם לֹא מָצְאוּ, כָּעִנְיָן שֶׁנֶּאֱמַר (ירמיה יד, ג) "וְאַדִּרֵיהֶם שָׁלְחוּ צְעִירֵיהֶם" וְגוֹ'. דּוֹרְשֵׁי רְשׁוּמוֹת אוֹמְרִים, לֹא מָצְאוּ מָיִם, דִּבְרֵי תּוֹרָה שֶׁנִּמְשְׁלוּ לְמַּיִם, שֶׁנֶּאֱמַר "הוֹי כָּל צָמֵא לְכוּ לַמַּיִם", לְפִי שֶּׁפָּרְשׁוּ מִדִּבְרֵי תּוֹרָה שְׁלֹשֶׁת יָמִים לְכַךְ מָרְדוּ. לְפִיכַךְ הִתְקִינוּ הַנְּבִיאִים וְהַזְּקֵנִים, שֶׁיְּהוּ קוֹרִין בַּתּוֹרָה בְּשַׁבָּת וּבְשֵׁנִי וּבַחֲמִישִׁי. כֵּיצַד, מַפְסִיקִין אֶחָד בְּשַׁבָּת וְקוֹרִין בַּשֵּׁנִי, וּמַפְסִיקִין בַּשְּׁלִישִׁי וּבָרְבִיעִי וְקוֹרִין בַּחֲמִישִׁי, וּמַפְסִיקִין בַּשִּׁשִּׁי. מֵעִיקָרָא תְּקִינוּ חַד גַּבְרָא תְּלָתָא פְּסוּקֵי, כְּנֶגֶד כֹּהֲנִים לְוִיִּם וְיִשְׂרְאֵלִים אָתָא עֶזְרָא, תִּיקֵּן תְּלָתָא פְּסוּקֵי כְּנֶגֶד עֲשָׂרָה בַּטְלָנִים.