3D tracking doesn't work well with D5100 with only 11 points, too little AF points across the screen, you are better off with auto area or single area AF-C

depends...

when you half press shutter button you see the red dot blinking, that's the point it will focus and 3D tracking use motion tracking to track the subject, if it detect the subject is moving it will move the tracking point forward and if your subject suddenly stop moving it might not be able to focus sometime, that's what i found out when i shoot motorsport last time

i also found that AF-C on our camera doesn't work well with subject moving towards our camera but works better for left right and up down panning, and 3D tracking AF-C check for focus every few millisecond which i find not very good for me so i normally use single point or auto area

if you are moving yourself with your subject then better use single point AF-C

AF-S is the best for still life, events and portrait, i use AF-C for sports and panning only

you might need to change the AF-C priority selection to focus if you are on release mode, focus priority the camera will make sure it is in focus before taking the shoots while release mode the camera will take the shoot no matter it is in focus or while it is focusing

D800 coming next
ETA this week with 85/1.4G

too bad not mine