What shoukd be my 3rd watch?

I think, for you, go with the GS!  You’ve got the classically identifiable Speedy icon to go traditional. The Milgauss is a slightly askew and off beat watch, so I would continue that trend with the pink dial GS. It‘s an under the radar eye catcher just like the Milgauss. 
 

Thanks for the recommendation. I figured that since I have a black and blue dial, something more white/silver should be in the list. I do agree that the GS is much more under the radar, not saying that the Cartier is flashy by any means, but certainly both can be dress up or down depending on the occasion.

That GS is stunning but it’s too big in my opinion. The Cartier is perfectly sized and it’s an icon. 

I chose B. I have the SBGA413 and want the Santos, though it's on the back burner at the moment. The Shunbun is my favorite and most worn watch, and even with the Santos, I think it would stay that way. It's more interesting, with the dial and the Spring Drive movement. It's titanium and has a date (which for me is a plus, though some may disagree). And Grand Seiko doesn't get enough credit for making robust, practical, everyday watches, especially with Spring Drive, which is less delicate than a standard mechanical movement. The Santos also costs a good bit more, and while I really like it and want it, part of me has trouble justifying the cost for what seems to me a simple three hand watch. Anyway, that's my thinking, if it helps yours. 

If you’re looking at the SBGA413, you should consider buying the SBGA443 instead. Exact same watch but re-released in Japan under a different reference number and would be cheaper overall. Useful info in the post and comments here: https://www.watchcrunch.com/K.evin/posts/total-steals-grand-seiko-sbga443-shunbun-and-sbga445-taitetsu-17690 

Of the two choices, the Cartier, but I'd go diver myself.

As much as I love the Santos, I'd suggest it may not be an ideal choice for a 3rd watch. I have the WSSA0037 model, and while I absolutely love it, it's a bit more polarizing than many other watches. There are days when I look down at my watch box and it screams out "wear me", and on those days, nothing else other than the Santos will do.  The moment I strap it to my wrist I get a big smile on my face, and I feel great wearing it. There are other days when my reaction is more like "anything else other than the Santos".  This same thing can happen with other watches in my collection, but never to that same degree. YMMV, but something to keep in mind.

I have both Santos 100XL and SBGA443 (SBGA413 from Japanese market) in my collection. Funny I am actually looking for a Speedy as my next watch to fill the spot as sports watch.

Generally I wear my Santos for work most of the time, and the Shunbun would be my afterwork/weekend casual watch either on bracelet or straps.

I would suggest going for the Santos if you have to pick one from the two.

No bad choices here, but the I prefer the stunning dial of that #grandseiko. The #cartiersantos is definitely an icon and it would be a great addition to your collection, but I’d wear the SBGA413 more so I chose it. Good luck making a tough choice!

Go with your heart, for me it was the GS. I have had the similar GS Winter. Four very happy years of ownership so far. 

I would go with the GS. I love my SBGA413 Shunbun 🌸 The dial captivates me everyday I wear it. I have found more joy in my Shunbun & White Birch than I have with all my Hans Wilsdorf Foundation timepieces. There are no doubt some drawbacks, but they can be easily overcome. Most people complain about the bracelet. Shunbun looks phenomenal on a leather strap! IMHO. Good luck and most of all enjoy your watch collecting journey! 🥂🤟

Seems like you are looking for "balance" in your collection when you pose the question in context of what you already have.  I would just recommend that unless you yourself feel the need to check off distinct boxes in the horological spectrum (i.e. tool watch, Chronograph, dress, GADA, disposable beater, super saturated color dial, blingy watch, subdued watch, etc) that you go with what you like and what you think you will wear the most.  Trying to fill in the blanks (in the proverbial watch box) invariably gets you thinking more in an external context rather than your own personal one.  At least that is my own personal experience.  :)  Teddy Baldassarre commented in one of his recent speeches that he no longer posts "My Watch Collection" updates for a similar reason, he only wants to be collecting for himself and not be influenced by thoughts of the community at large or trends and patterns.

Having said all of that, I am a personal owner of the Shunbun, and it is gobstakingly beautiful!  The pink is very subtle and only visible at certain steep angles and sometimes affected by ambient lighting.  The majority of the time you see the incredible silver patterns on the dial.  From that standpoint it is always a pleasant surprise when it does show itself.  And it can look good on so many straps.  (I really do have a "strap problem" since having bought that GS!).