Like in tf2, I'm wanting to use the technique of displaying bullet damage in an FPS game by way on 2d text above where the bullet hits the target in 3d like so:

My only problem is that I don't know how to translate this 3d position of where the bullet hit into 2d. Any ideas?

Most frameworks or libraries have a method to transform a 3dvector.

Vector3 temp = Vector3.Transfom(3dposition, view * projection);
 
Vector2 screenPosition = new Vector2(temp.x, temp.y);

You can use picking + time of impact just as @cozzie described above to estimate the contact point.

A good tutorial: http://www.rastertek.com/dx11tut47.html