3. Given $\tan (\alpha-\beta)=\frac{2}{5}, \tan \beta=\frac{1}{2}$, then $\tan (\alpha-2 \beta)=$
A. $\frac{3}{4}$
B. $\frac{3}{8}$
C. $\frac{1}{12}$
D. $-\frac{1}{12}$
3. D $\tan (\alpha-2 \beta)=\frac{\tan (\alpha-\beta)-\tan \beta}{1+\tan (\alpha-\beta) \tan \beta}=-\frac{1}{12}$