וַיַּֽעֲשׂ֥וּ בְנֵֽי־לֵוִ֖י כִּדְבַ֣ר מֹשֶׁ֑ה וַיִּפֹּ֤ל מִן־הָעָם֙ בַּיּ֣וֹם הַה֔וּא כִּשְׁלֹ֥שֶׁת אַלְפֵ֖י אִֽישׁ׃
And the sons of Levi did according to the word of Moses; and there fell of the people that day about three thousand men.
Read kabbalistic (mystical) interpretation of Exodus 32:28: and classical mystical sources.