I was studying from Schroeder's thermal physics book. When it talks about Einstein solids it says that they have 2 degrees of freedom thus $U=NkT$
However, I thought when we talk about Einstein solids, we think about harmonic oscillator that oscillates in 3 dimensions. Thus I was expecting Einstein solids to have $U=3NkT$ degree of freedom (3 from position space 3 from momentum space).
What is the correct interpretation of 2 degrees of freedom in Einstein solids?
Note: I might be making a big mistake as I have not taken thermo course in my degree yet, and I am studying all by myself.
Note 2: I know that my approach is flawed as $U=NkT $ result can be obtained from entropy function of Einstein solid. I'm just trying to use Equipartition theorem