Musar do Wyjścia 32:35

In order to solve the various remaining difficulties, let me first explain the meaning of the Yalkut as well as the statement that Moses had deducted 3,000 who had been killed by the Levites during the operation referred to in Exodus 32,27-28. These three thousand people were the ones against whom there were valid witnesses, and who had been warned of the consequences if they persisted in what they were doing or were about to do. Further there was a number of people who died through a plague at the hands of G–d, because they could not be brought to trial legally. The Torah refers to these people in 32,35 without specifying their number. In view of this why did Rabbi Hoshiya in the Yalkut say that only 3,000 had died? Why does he give us the impression that only 3,000 were missing from the number of eligible young men prior to the sin of the golden calf? It certainly seems unlikely that those who died from the plague were all under twenty years of age, or that they were all part of the mixed multitude. It seems certain therefore that a number of thousands of young men of the age of 20 and over, all of whom belonged to the twelve tribes, died as a result of that plague. We must assume that the number of young men who reached their 20th birthday before the first of Tishrey in that year made up for those who had died as a result of the plague. Hence, once Moses knew the number of Jewish men of military age prior to the sin of the golden calf, he was able to account for the number who had died during this episode.