Whom did the Levi'im kill?
Refer to 32:20:1:1* & 3.
What are the implications of the word "ki'Sheloshes Alfei Ish"?
Kol Eliyahu and Divrei Eliyahu: It implies that only the men worshipped the golden calf, but no women. 1
Kol Eliyahu and Divrei Eliyahu: As the Pasuk hints in Koheles 7:28"Adam Echad me'Elef Matzasi ve'Ishah be'Chol Eileh Lo Matzasi" - 'One man out of a thousand sinned by the Eigel (Refer to 32:28:151:1) when they announced "Eileh Elohecha Yisrael", but not one woman'. See Torah Temimah, note 39, citing Midrashim who implies that some women did serve the Eigel.
What is the significance of " ... three thousand men"?
Kol Eliyahu and Divrei Eliyahu: Although only six hundred men who sinned (one in a thousand) 1 three thousand were killed (five-fold). Hence the Midrash cites the Pasuk in Mishpatim 31:27 "Chamishah Bakar Tachas ha'Shor." 2 If only 600 sinned, why did 3,000 die? It is because they blemished the Shor in the Merkavah; for this, they paid five-fold.
Even after people died due to the Eigel, six hundred and three thousand five hundred and fifty men remained, and some say that this excluded men below the age of twenty and above the age of sixty. (Some say that it excluded also the tribe of Levi. We know that no Levi'im sinned, but perhaps they affected the count for 'one out of a rhousand). Perhaps it says approximately three thousand because it was slightly more (PF).