Am I missing something here?
In your truth table there are only three bits, that is all you need to count to 7. Why do you need four bits?
So you will need three D type flip flops to implement a 3 bit counter.
If you use four D types and build a four bit counter, you can count up to 7 by just ignoring the fourth bit.
I guess the present state can be determined by looking at the states of the Q outputs of the flip flops, that is what most people do I think..
As for the next state, I guess that it will be the current state +1, except when the current state is 7, then it will be 0.
Your truth table may be better written as:
truth table
N C B A
--- -------------------
0 0 0 0
1 0 0 1
2 0 1 0
3 0 1 1
4 1 0 0
5 1 0 1
6 1 1 0
7 1 1 1
The least significant bit is usually denoted by A.
JimB