How to write video from RCA to micro SD using Arduino?
Simple you can't. The arduino is not fast enough and does not have enough memory.
Is a composite camera always an NTSC camera?
No there are PAL composite outputs as well.
is this the same for all NTSC cameras with just three wires with a similar configuration of power, ground and video?
Nearly, this would apply if it were a colour camera, if it's black and white it aint NTSC ( it stands for Never Twice the Same Colour 
)
can a composite camera have more than three wires
The word composite gives the clue, the other sort you can have is component with a separate signal for R, G, B & sync, there is also S-video plus a lot more, see:-