In yesterday’s Parsha, there is a reference to the family of the Leviim. They had their own separate counting from one month and onwards.
In the middle of this section, אהרון is mentioned with a number of dots on top of his name. The explanation for these dots is to tell us that Aharon was not counted as part of the Levi family. There were 22,000 לויים that did not include the Kohanim.
Aharon was kept separate because of his importance and holiness. His role as אוהב שלום, a lover of peace, and his loyalty to Moshe, where he showed no jealousy whatsoever, put him in a special category. The dots above his name in Chapter 3:39, was meant to emphasize this.