I have just learnt the basics of the EX3 and use it on auto focus.
When I film a group of people moving fast eg running, and I want to freeze frame in FCP, one person in the centre is in focus and the rest are out of focus. How do I change the settings on the EX3 so everyone is in focus?
Don't use auto focus.
The art and science of tracking focus is an important skill for camera operator / DP.
The wider the aperture (f.1.9 for example) the shallower the depth of field potentially.
The EX has a bar at the bottom of the display which shows you focus distance and the width of the marker shows you how wide/narrow the range is approximately. You might want to keep the aperture closer to something like f 5.6 but you're balancing aperture with your lighting circumstances. If it's too dark at f 5.6 you'd have to do something other than open the aperture. You might be faced with compromises like turning up gain. Slowing the shutter speed gives you more light but the added motion blur is probably NOT what you want when doing slo mo.
I should also mention that zooming in with the camera can also result in what appears to be shallower depth of field. You'd be better off moving physically closer to the subjects. I can imagine that some might want to consider changing lenses as well if you have options.