Hi thefonz
I have the D90 and as the manual says 3D tracking is really designed for recomposing your shots and not for tracking fast moving objects , tried it a couple of times with my white Jack Russell in a large meadow in very good light . I used the centre focus point to get a good lock on him and as soon as he ran off I panned the camera to follow him and the camera remained locked on the patch off grass that he was standing on but if I focused on a stationary object and moved the camera around you can see in the view finder each focus point picking up the lock as the object passed by .
If your going to ask me why it's called 3D tracking I'm afraid I haven't got the foggiest
However the dynamic area A/F combined with A/F C and high speed continuous shooting is amazing , I promise you it will put a smile on your face when you see how well it works
I do use auto ISO and usually app priority but unless I'm doing something like macro work I use the dynamic and AF C setup .
I know the 5100 has a much newer system than the 90 so you may have better luck with the 3D than I did , I had read reports that the 300s with its better metering and more numerous focus points would track successfully using the 3D so I would be very interested in your experiences .
Good luck
Mark