Significant figures are calculated through certain rules:

(i)  All the digits that have no zero are significant. 

(ii) Zeros that are written at the right hand side of the last non-zero digit in a number with the decimal are significant, while if the decimal is not present then the zeros are not significant. 

(iii) The zeros that are written before non-zero digits are not significant. 

(iv)  Zeros that are written between the non- zero digits are significant. 

So, here number of significant figures in $A$, $B$ and $C$ are four. Thus, $N_A=N_B=N_C$.