There are 3 different punishments stated for the sin of the Golden Calf, two of which are ambiguous in nature. The most explicit is that Moses and the Levites kill about 3,000 men: “kill each man his brother, each man his neighbor, and each man his relative.”
The text goes a far way to explicitly present the Levites in the favorable role of expiating the sin. In other words, the Levites are portrayed in a priestly function here, making atonement for the sin by expunging the sinners. Ironically this portrait of the Levites as Yahweh’s means for expiation, and conversely the negative portrait of Aaron, is negated by the later Priestly material in Exodus 25-31 and 35-40 that now encases this Elohist narrative. In the Priestly texts which surround the Golden Calf narrative, Aaron is in the midst of being selected by Yahweh as chief priest, that is he who resides and officiates over all atoning sacrifices, and conversely the Levites are given a remedial position in the cult (#160-161)—a testimony to the power of redaction.
The other 2 punishments are recorded in verses 34 and 35. Verse 35 is the more ambiguous, claiming that Yahweh has already struck them down, but the text does not specify how, perhaps by a plague, one of Yahweh’s favorite forms of punishment.
The other punishment is recorded as a future event in the narrative, but which most likely is an event that has already occurred from the writer’s vantage point. Building off of the previous entry where we learned that the Golden Calf episode is really a polemic against Jeroboam’s bull altars at Dan and Bethel (#157), and both writers refer to it as “the great sin,” it is only reasonable to see the day of reckoning for this sin as the historical destruction of Israel by the Assyrians in 722 BC. As the Deuteronomist asserts:
And it was so, because the children of Israel had sinned against Yahweh their god, who brought them up out of Egypt from under the hand of Pharaoh king of Egypt, and had feared other gods, and walked in the statues of the nations whom Yahweh cast out from before the children of Israel… and they set up pillars and Asherim upon every high hill and under every leafy tree… and they served idols… and made molten images, even two calves… that Yahweh was very angry with Israel, and removed them out of his sight. There was none left but the tribe of Judah only. (2 Kgs 17:7-18)
For he rent Israel from the house of David, and they made Jeroboam the son of Nebat king, and Jeroboam drew Israel away from following Yahweh, and made them sin a great sin… So Israel was carried away out of their own land to Assyria. (2 Kgs 17:21-23)
There can be little doubt that Exodus 32:34—Yahweh will reckon their sin on the day of reckoning—is an allusion to the 722 BC destruction of Israel, the byproduct of Jeroboam’s sins—that is his calf altars. At least this was how the southern pro-Solomonic Deuteronomist presented history.
This isn’t a contradiction. There are simply multiple punishments. They are not simply punishments for just worshiping the idol (God chose not to punish them for this in 34:10-14), but punishments for turning their backs on God (34:26 only the Levites stood with God). Obviously, not all of the men who fell into idol worship were slain (only ~3,000 were), so the rest were punished by a plague (34:35), and later punished by some other means (34:34).