I have never shot an MMA but did a lot of ballroom dancing shoots. I put the D3 in AF-C, 51 point, Auto Area with AF-On as focus initiator. However, when you first start focusing, your subject's eyes must be within the group of focus points, ideally in the middle. Once focus is acquired, you can then keep you finger on the AS-On button and recompose. The camera will track. It is actually uncanny how well it tracks. It will focus on your subjects' eyes instead of other body parts. So if a dancer was pointing a finger at me, the D3 would focus on her eyes instead of her finger tips. However, your lens choice will make a difference at how fast the cameras tracks as well. The 24-70 should be fine. My 70-200 is fast but cannot compare with the 200 f2. I hope this helps.
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