Why do the targumim to Sh'mos 32:27 use g'var twice and then enash?
The pasuk in question: לב,כז וַיֹּאמֶר לָהֶם, כֹּה-אָמַר יְהוָה אֱלֹהֵי יִשְׂרָאֵל, שִׂימוּ אִישׁ-חַרְבּוֹ, עַל-יְרֵכוֹ; עִבְרוּ וָשׁוּבוּ מִשַּׁעַר לָשַׁעַר, בַּמַּחֲנֶה, וְהִרְגוּ אִישׁ-אֶת-אָחִיו וְאִישׁ אֶת-רֵעֵהוּ, וְאִישׁ אֶת-קְרֹבוֹ.
and the Targum: וַאֲמַר לְהוֹן, כִּדְנָן אֲמַר יְיָ אֱלָהָא דְּיִשְׂרָאֵל, שַׁוּוֹ גְּבַר חַרְבֵּיהּ, עַל יִרְכֵּיהּ; עֵיבַרוּ וְתוּבוּ מִתְּרַע לִתְרַע, בְּמַשְׁרִיתָא, וּקְטוּלוּ גְּבַר יָת אֲחוּהִי וּגְבַר יָת חַבְרֵיהּ, וַאֲנָשׁ יָת קָרִיבֵיהּ.
I don't have a ready answer to this variance. However, I can point out that the Samaritan Targum lacks this difference, and has גבר for each. Targum Pseudo-Yonatan has the divergence. Except that this Tg Pseudo-Yonatan also has the divergence, or maybe not. They apparently have both nuschaot, and someone selects אנש as the correct one.
Similarly, for Onkelos, I am not entirely convinced that אנש is the correct text. If we look at dfus Savyonita, from 1557, we find the following in Onkelos:.
I underlined וגבר in red. So too in this Chumash, from Bologna in 1482.
So we really should investigate as best we can manuscripts of Onkelos to determine that the phenomenon is real, before setting out to explain the phenomenon. Tg Yonatan might have been bleed-over from texts of Onkelos, so I wouldn't take it as such strong evidence. But the Peshitta seems to be much stronger evidence.
Off the cuff, my inclination is that אנש is used as a translation of person, while גבר is used as a translation of male person. I can see how each would fit well in context. We are saying that each and every person should take X action, so I would expect אנש. But on the other hand, the people involved are warriors, and thus are surely males, so I can understand גבר as a replacement. My inclination is that Onkelos selected only one of these, but some text emended all of these, and someone wrote אנש as a marginal gloss once, to indicate the general divergence, which was then misinterpreted to mean a replacement of only one of these. I cannot prove it, though, and צריך עיון.