Actually, the Rokinon/Samyang/Bower 85mm is EF mount, so you can use it on your 5d3. It'll just be 85mm portrait length, rather than the ~136mm equivalent short telephoto you are used to. The Tamron 17-50, doesn't look like you can.
As for what lens(es) you may need, how do you usually shoot? Did you tend to stick at 1 or 2 focal lengths on your 17-50? Or were you all over the place? If you want a short telephoto, the Canon 135L is a great lens. Otherwise, the Tamron 24-70 is a very good lens. Not quite as good optically as the Canon 24-70 v2, but still quite good. And a lot less expensive.
Do you want to be able to zoom in/out during shooting? If so you should look at investing in a true cine lens, as they are
parfocal while the vast majority of still photography lenses are not.
Otherwise, what's your preferred poison? Zoom lenses won't be faster than f/2.8, but you'll often be stopped down anyway to get the DoF you need. Especially on a FF 35mm sensor. However, as usual, primes tend to be sharper and have better IQ than zooms for a lesser price.
As for audio, the 5d3 does have an audio-out jack as well as in, however if you want to get into the MagicLantern RAW video, you'll need an external recorder as it won't record any audio at all so you'll need some kind of clapboard or similar to sync up the audio in post.
