I would recommend against renting a camera that you've never used before for this wedding - especially one that is so different from your 7D. The last thing you want to be doing is worrying about settings and focus systems and wondering how the images will actually look once you put them on your computer.
My first week of using the 5D3 was a big learning experience. I found the ISO's I could use in different lightings in order to get good looking shots. I figured out which AF systems I wanted to go with during different situations.
For the first week I had it, I was getting held up thinking about settings. After a few months, it became second nature and I don't have to spend any extra time fiddling with buttons because I know what things will look like, and I know where the buttons are.
Also, the metering is a little different on the 5D3 than other Canons that I have used. I find that it tends to underexpose, and the histogram confirms this. If there is harsh contrast you will almost always have to bump up the exposure compensation nearly +1. It's like the 5D3 won't allow for ANY blown highlights when set to 0 - not even a single street light. This might not sound like a lot, but unless you're shooting in full manual all the time be ready for this.
Then there is the problem with lenses. I know people might yell at me about this, but the nifty 50 looked like garbage on FF in certain situations. If there is any harsh contrast you are going to get HORRIBLE chromatic aberration - and I do mean horrible (even when stopped down). It was beyond repair in LR. I loved my nifty-50 on crop, but I ended getting rid of it and got the 1.4 instead and it's like a different galaxy. My point is that at least one of your lenses will act very different because more of the glass is included in your picture. I used the 24-70 on it for a wedding I just did, and I made sure to spend about 3-4 days with it before bringing it on a gig - and I'm glad I did because I found some sweet spots after trying everything. I wouldn't have been able to do this if I just tried it for the first time that day.
I remember having a conversation with a friend after I had my camera for about a month, and what I literally talked about was how I would have been in deep dookie if I rented a 5D3 and used it for a wedding before spending a few weeks with it. I would have missed moments fooling with things, I would have had blurry or noisy shots because I wouldn't have been sure what ISO's would look good in certain lightings, and certain shots would come out like garbage because I would not have been taking into account how my lenses act on FF.
If you feel comfortable with a new camera, different layout, different focus systems, different capabilities, and literally something that makes your pictures look different (crop vs. FF) then by all means rent it - it's an incredible camera. But my honest suggestion would be to either use it for at least a few days and try it in just about every setting you can think of before the wedding or just stick with your already sweet 7D and just rent some more glass. I'm sure you will end up just renting it, and I'm also sure that you'll get some amazing shots - but you'd double the amazing shots if you used it for a week or two before taking it into battle.