When you go to the MS store, they have 2 screen protectors on the shelves. Both are glass, and rather expensive. But they are the only once that the sales staff recommend. I have no clue how good they are or what they are like.
I bought the Microsoft one when I ordered my second SP3 - after fitting (and having problems) with a cheaper one on my first SP3.
Microsoft one is more expensive, but for me, well worth the price difference. Went on a breeze, no difference in touch control of screen and holes cut perfectly for camera and speakers.