What's the difference between Blu-ray and Blu-ray 3D BD+BD+Digital HD

Multiple disks in this case, you can just watch the 2d version
EDIT:I know i've seen 3D only blu rays, those wont play on non 3D TV/Monitors, for obvious reasons (or i guess on PC you might be able, by choosing either left or right frame)

well that second option has 3d bluray, standard bluray, and a digital format; I usually get the bluray/digital combos since I personally do not care for the 3D gimmick. (visually impaired does not look different to me)

Let's say I'll get 3d glasses in the future so if I order Blu-ray 3D BD+BD+Digital HD then will I still be able to watch it on my PC monitor without any (3D) glasses?

to watch a 3d movie you need a 3d blue ray player, a 3d tv, a good enough hdmi cable, a 3d blue ray disk and 3d glasses. if only one is missing, ofc you won't have your 3d movie.

(3D BD+BD+Digital HD)

Breaking it down:

3D BD = The 3D copy of the movie, on a 3D Bluray disc
+
BD = The 2D copy of the movie, on a Bluray disc.
+
Digital HD = 2D digital copy. In this case, since this is a Disney movie, that means it's in your DisneyMoviesAnywhere account and streaming provider of choice (Amazon, iTunes, Google Play, Microsoft, Vudu). Other studios sometimes provide iTunes only, or Ultraviolet versions.

Bottom Line: Yes, you will be able to watch the movie in 2D, and then 3D eventually with that disc.

Generally, these notations mean multiple copies of the movie on multiple discs. Same goes for if you see something like: BD + DVD + Digitial HD or even 3D BD + BD + DVD + Digital HD. Either of these would throw in a DVD disc on top of the Bluray disc(s).