Distinct eigenvalues of a normal matrix

Given a matrix $X\in\mathbf{R}^{3\times n}$ and a diagonal matrix $D$ with known positive diagonal entries, what conditions may I impose on the columns of $X$ such that $XDX^T$ has distinct eigenvalues?

Here are some things that I already know:

If $D=\text{diag}(a)$ and $a\in\mathbb{R}^n$ , then

$XDX^T=\sum_{j=1}^na_jx_jx_j^T$ .

Also, this seems to be the most promising lead I have:

$\lambda_i=\sum_{j=1}^na_j(x_j^Tv_i)^2$ ,

where $\lambda_i$ and $v_i$ is an eigenvalue/eigenvector of $XDX^T$ .

Because $XDX^T$ is Hermitian, the eigenvectors define an orthonormal basis. Thus the change from the canonical basis to a basis of the eigenvectors is solely a rotation which does not change the geometry of the vectors $x_j$ . Therefore, it seems that any geometrical properties that I might find before the change of basis might still hold, but I don't know how I can prove this.

Thanks