Upload & Sell: On
If you are hand holding, and do not consider live view as practical, you have two methods to consider besides the actual viewed sharpness on the viewfinder screen. If the camera is in single shot, and you move the focus ring slowly, focus points can "light up", indicating that point is sharp at that moment. But you mentioned the 5D3, so that must not be working out for you because of the illumination issues. I can tell you that it works fine in low light on the 5D2.
The other very practical option is to estimate the subject distance, and then use the focus scale on the lens to set foucs. Unless the action is too fast, estimated distance works very well for this wide angle lens. You also have the added bonus of a very usable depth of field scale right on the lens that combines with the long throw of the focus ring to allow very practical use for determining depth of field.