Why does the verse tell them to go to Beis Kel and sin?
Rashi: This is like one tells a lowlife 'do (Aveiros), do, until your measure [for sin] is full.'
Radak: This is not a command that Hashem wants them to do. Rather, it is a threat, like "Semach Bachur b'Yaldusecha [?Ki Al Kol Eleh Yevi'acha Elokim ba'Mishpat]" (Koheles 11:9). It mentions Beis Kel, for the calf was there.
Malbim: Now it explains the fourth introduction "Im Yitaka Shofar ba'Ir v'Am Lo Yecheradu" (3:6). Hashem commanded the Nevi'im to sound the Shofar, tell them their sin, and inform them of the coming punishment, and they did not fear. The Nevi'im told them in the name of Hashem "come to Beis Kel and sin; ? v'Gam Ani Nasati Lachem Nikyon Shinayim" (6) ? just like you sinned greatly, I will punish greatly! "Hashmi'u" (5) refers to the entire matter. You announced to go to Beis Kel and sin!
Why does it mention "Gilgal"?
Radak: This is like it says "Kol Ra'asam ba'Gilgal", "ba'Gilgal Shevarim Zibechu" (Hoshe'a 9:15, 12:12). The Mishkan was in Gilgal initially after Yisrael crossed the Yarden. That place was chosen. Nevi'ei ha'Ba'al said that they should serve Ba'al there, since it is chosen. It says "Harbu Lifsho'a" ? it is not enough that you fixed idolatry in Beis Kel. Also in Gilgal you sinned greatly!
Why does it say "v'Havi'u la'Boker Zivcheihem"?
Rashi: Priests of the Ba'al tell them, it is an easygoing idolatry. It is not stringent upon you. The Torah said "Lo Yalin la'Boker" (Shemos 34:25), but for idolatry you can bring in the morning. Also refer to 4:4:4:1, 4:5:1:2, 4:5:2:2.
Radak #1: You switched My Avodah to other gods. Just like you should have brought your Korbanos in My house, you bring them in a house of idolatry. "La'Boker" is every morning ? you bring your Korbanos there every day.
Radak #2: They offer Shelamim before Olah. The Torah said oppositely ? the Olah (morning Tamid) is before all Korbanos!
Malbim: They announced that they should bring Korbanos to idolatry every morning.
What do we learn from "li'Sheloshes Yamim Maseroseichem"?
Rashi: [Priests of the Ba'al say,] the Torah said about Kodshim Kalim "b'Yom Zivchachem Ye'achel umi'Macharas" (Vayikra 19:6); idolatry allows eating Ma'aser Behemah for three days.
Radak: "Yamim" is years, like "Yamim Tihyeh Ge'ulaso" (Vayikra 25:29). This is like the Torah said "mi'Ktze Shalosh Shanim Totzi Es Kol Maser Tevu'asecha" (Devarim 14:28). I.e. all matters of Korbanos and Maseros, bring them to the idolatry house, like you should have brought to Me.
Malbim: They announced that they should bring tithes every three days.
