3. Let $f(x)=\frac{1+x}{1-3 x}, f_{1}(x)=f[f(x)], f_{2}(x)=f\left[f_{1}(x)\right], \cdots, f_{n}(x)=f\left[f_{n-1}(x)\right], \cdots$. Then the value of $f \ldots$ (4.7) is
A. $\frac{37}{57}$
B. 4.7
C. $-\frac{57}{131}$
D. 2005
3. $\mathrm{A}$ Hint: $f_{1}(x)=\frac{x-1}{1+x}, f_{2}(x)=x, f_{3}(x)=f(x), f_{4}(x)=f_{1}(x) \cdots, f_{2005}(x)=$ $f_{668 \times 3+1}(x)=f_{1}(x)=\frac{x-1}{1+x}$.