I have more than what I need to take amazing pictures in almost any situation but I after I buy something I find myself looking to the next thing to buy. I don't know what is wrong with me it is like a disease. I know I am crazy and should just be content with what I have. So if you were in my situation and you were going to buy something new would you sell one of the 5D3s and get a 1DX or would you buy the 85mm 1.2? I am a little afraid the 85mm 1.2 will be a little slow focusing at receptions similar to the 50mm 1.2.
Well, I think most of us here feel exactly the same way. I know I do!
I'm not a Wedding Photographer, but I was a Wedding Videographer for a few years, so I can relate to the importance of capturing those once in a lifetime photos. There's no do-over. I have the 85mm f/1.2L II and I shot my friends Wedding as a second shooter, but unaffiliated with the wedding photographer, so I was able to play around and let me tell you I would not trust the 85mm 1.2 for critical moments during the ceremony or reception, but it definitely can find a place in your kit, for portraits, candids, rings, non-critical reception shots, flowers and things like that.
Of course you can nail great shots with or without pre-focusing and I have, but I wouldn't trust it down the Aisle. It's heavy, the DOF is thin, and it's slow, but when you do nail it, it's spectacular.
The 1DX is obviously an Awesome Camera and I can certainly see you selling one of your 5DIII's for it, but it's also a bit heavy to shoot with all day, and you should also consider that if you shoot with two cameras at the same time, then switching from the 1DX to the 5DIII quickly may present some confusion, and the 1DX will definitely feel heavier on your shoulder, especially with the 85mm f/1.2L attached.
However, I really do love the 1DX and 85mm f/1.2L II. I'm not trying to talk you out of either. I'm just adding some of my thoughts for you to consider.
If I did have to pick one then I would recommend the 85mm 1.2L II.