I've used MF lenses on my 7D, here's my advice:
1 Get a Katz-Eye screen. It's $200 or so, but well worth it, with the split-rangefinder you can actually see where you're focussed, and the rest of the screen shows a much brighter image, with true DOF. If there's none available for 5D3 (i haven't checked), or it's too expensive, then:
2 Use Live-view. Tripod is best, but i've done it handheld with an 55 f/1.2. Zoom in to 5x and you can see if you're focussed well enough. Don't forget to zoom back out to 1x before clicking the light-reading though (if you're zoomed in and the lighting changes, you could end up with an all black or all white photo). Not the best for action.
3 Hyperfocal. For 35mm or wider, and f/4 or higher (narrower), just focus on something 10 meters away or so, and everything will be in focus from 5m to infinity, you only have to change for closer subjects.
As for the 'confirmation light', as far as I know you Do get it with Zeiss (ZE) lenses. For everything else (like Samyang, and/or lenses from any other brand on an adapter), you don't. But then you can buy a $20 chip from 'ml-gvalt' on ebay and glue it on yourself...