In that case, yes, it's perfectly possible.
There's 4mm to play with between R and EF mount. I've adapted Contax C/Y and Olympus OM lenses to my 5D in the past before I moved to Sony FE, which is an easier platform to use with adapted lenses. R is an SLR mount, so you shouldn't have too many problems with light falling at the wrong angle onto the 5D's sensor.
You might have to watch out if some of your lenses have a rear element or other part of the mount that protrudes backwards - there is a risk that the 5D3's mirror will hit it (AKA mirror strike). Not
so likely actually to damage anything badly, but it may prevent your taking pictures. Is certainly an issue with some C/Y lenses (it tends to be worse with wide angle retrofocus designs IIRC) .
IME whether that's a problem very much depends on the exact lens and camera body combinations you may end up with. It may not be a problem for you at all, but I would recommend doing some research into what you have.
You might try looking at Fred Miranda's
Leica and alternative forum where there is a wealth of experience doing this over the last fifteen years or so.
e.g.
https://www.fredmiranda.com/forum/topic/895186/0
If you are looking to do it seriously, I'd recommend Novoflex adapters as they have always been utterly bulletproof for me. They are quite pricey but my experiences with other brands have been more mixed. Fotodiox may be worth a look at if you want to spend a bit less. YMMV vary on this - with such a small difference in the register distances between the two mounts you're not so likely to suffer from internal reflections in the adapter appearing in your images, which is a problem I have run into with cheaper brands.
Manual focus with any DSLR (or autofocus film body) is a bit of a challenge. The viewfinders aren't really designed for it and they lack niceties like a split-prism or fresnel focus aids you might have with a native body. Obviously, this is easier to deal with if you're shooting at f/8 rather than f/1.2.
As for 'is it worth it?' question: Your Leica lenses should be able to deliver good results and certainly it's a lot of fun bringing an old lens back to life.
