Speak to the children of Yisra᾽el that they take to me an offering: of every man whose heart prompts him to give you shall take my offering.
תקחו את תרומתי YE SHALL TAKE MY HEAVE OFFERING — Our Rabbis said: the expression תרומה is used here three times, being an allusion to three different heave offerings; one is the heave offering which consisted of a beka (half a shekel) a head, and of which the sockets were made, as is set forth in the section אלה פקודי (Exodus 39:26-27); another is the heave offering for the altar — a beka a head that was given to the funds (more lit., “the basket”, in which collections for communal or charitable purposes were made) from which to purchase the communal sacrifices; (see Rashi on Exodus 30:15) and the other one is that implied in the word תרומתי “My heave offering” and referred to by the word כסף in the next verse — the heave offering for the Tabernacle which was a free-will gift from each individual (Jerusalem Talmud Shekalim 1:1). Thirteen different articles (cf. Tanchuma) that are mentioned in this section were all required either for the work of the Tabernacle (i.e. the construction of the Tabernacle or making the articles contained therein), or for the priests’ garments, as you will find when you look closely into the matter.
Why does this parsha only detail the last donation? (The donations for the sockets is detailed in parshas Pekudei, and the donations for the communal sacrifices, in Ki Tisa)
Also, the terms used for each of these three donations are slightly different:
Sockets: "Take to Me an offering"
Communal Sacrifices: "Take My offering"
Free-will gift: ""This is the offering"
(א) וְזֹאת הַתְּרוּמָה אֲשֶׁר תִּקְחוּ וְגוֹ'. זָהָב, כְּנֶגֶד מַלְכוּת בָּבֶל, דִּכְתִיב בָּהּ: אַנְתְּ הוּא רֵאשָׁה דִּי דַהֲבָא. (דניאל ב, לח). כֶּסֶף, כְּנֶגֶד מַלְכוּת מָדַי, דִּכְתִיב: וַעֲשֶׂרֶת אֲלָפִים כִּכַּר כֶּסֶף (אסתר ג, ט). נְחֹשֶׁת, כְּנֶגֶד מַלְכוּת יָוָן, שֶׁהִיא פְּחוּתָה מִכֻּלָּם. וְעֹרֹת אֵילִם מְאָדָּמִים, כְּנֶגֶד מַלְכוּת אֱדוֹם, שֶׁנֶּאֱמַר: וַיֵּצֵא הָרִאשׁוֹן אַדְמוֹנִי (בראשית כה, כה). אָמַר הַקָּדוֹשׁ בָּרוּךְ הוּא: אַף עַל פִּי שֶׁאַתֶּם רוֹאִין אַרְבַּע מַלְכֻיּוֹת מִתְגָּאוֹת וּבָאוֹת עֲלֵיכֶם, אֲנִי מַצְמִיחַ לָכֶם יְשׁוּעָה מִתּוֹךְ שִׁעְבּוּד. מַה כְּתִיב אַחֲרָיו, שֶׁמֶן לַמָּאוֹר, זֶה מֶלֶךְ הַמָּשִׁיחַ, שֶׁנֶּאֱמַר: שָׁם אַצְמִיחַ קֶרֶן לְדָוִד עָרַכְתִּי נֵר לִמְשִׁיחִי (תהלים קלב, יז).
(1) This is the offering which ye shall take of them: gold, etc. (Exod. 25:3). The gold corresponds to the kingdom of Babylon, as it is written concerning her: Thou art the head of gold (Gen. 2:38); the silver corresponds to the kingdom of Media, as is written: And I will pay ten thousand talents of silver (Est. 3:9); and the brass corresponds to the kingdom of Greece, which was the least (powerful) of all. And rams’ skins dyed red corresponds to the kingdom of Edom, for it is said: And the first came forth ruddy (Gen. 25:25).10A play on words, linking Esau who was born ruddy (adom) with Edom. The Holy One, blessed be He, said: Even though you suffered because of the four arrogant kingdoms that attacked you, I will bring salvation unto you from the midst of servitude. As it is written: The oil for the light, (Exod. 25:6). This refers to the Messiah, as it is said: There will I make a horn to shoot up unto David, there have I ordered a lamp for Mine anointed (Ps. 132:17).
The Midrash explains that the four materials used to build the Mishkan correspond to the four exiles.
How could materials that are used to build the Mishkan allude to four kingdoms that destroy it [the Beis HaMikdash]?
The spiritual service of constructing the Mishkan is divided into three categories:
(ב) שִׁמְעוֹן הַצַּדִּיק הָיָה מִשְּׁיָרֵי כְנֶסֶת הַגְּדוֹלָה. הוּא הָיָה אוֹמֵר, עַל שְׁלשָׁה דְבָרִים הָעוֹלָם עוֹמֵד, עַל הַתּוֹרָה וְעַל הָעֲבוֹדָה וְעַל גְּמִילוּת חֲסָדִים:
(2) Shimon the Righteous was one of the last of the men of the great assembly. He used to say: the world stands upon three things: the Torah, the Temple service, and the practice of acts of piety.
The simple reason why one donation would not have worked is because the donations were (also) used to take a census of the Jewish people, and this would only be possible if everyone would separately donate the same amount.
The inner reason for this, is because there are 3 ways to elevate the world...
תקחו את תרומתי. ויתכן לפרש כי מה שהזכיר בכאן שלש תרומות, ויקחו לי תרומה, את תרומתי, וזאת התרומה, שהם כנגד שלש תרומות שהעולם נתרומם בהם, והם החכמה והתבונה והדעת
It is possible to explain the fact that the word תרומה occurs here three times in two verses as meaning that the Torah speaks of three different contributions by means of which the world is elevated. They are: חכמה, תבונה, ודעת, “wisdom-insight-know-ledge.”