The key to pulling a good key is a clean separation between key and subject. All things being equal, higher resolution will give a sharper edge between key and subject, which is good. But that's just one consideration. Chroma subsampling is another. Chroma recorded at full bandwidth (at least 10-bits and 4:4:4) will give a cleaner key than chroma recorded at reduced bandwidth, such as 4:2:2, or worse, 4:2:0. Chroma recorded at 4:2:0 is 1/4 the total resolution of chroma recorded at 4:4:4. This not only fattens up the key's edge, but averaging of four chroma samples creates mystery colors that never existed in the original image, further confusing the key. The EOS 5D Mk III can be rooted and rebooted with Magic Lantern software to create truly great RAW video, but the stock camera's video uses 8-bit 4:2:0 Chroma subsampling. It is possible to use HDMI out to record 8-bit 4:2:2, but you have to record audio separately in that case.
The thing to remember about green screen is that unless you have a major sound stage with 60' ceilings and 100'+ wide scrims, you will probably be doing all your work against a small background in a small room, which means very little flexibility in terms of both working distance and focal length. Which also means that a fixed-lens camera like the DJI Osmo 3 will either qualify for that environment or be radically unqualified. But the 5D Mk III is a very expensive camera not terribly well-suited for green screen work. You can go much cheaper with a GH4 (which can record 10-bit 4:2:2 1080p internally).
