jmc

If $a>0$ and $b>0,$ a new operation $\nabla$ is defined as follows: $$a\nabla b=\frac{a+b}{1+ab}.$$For example, $$3\nabla 6=\frac{3+6}{1+3\times 6}=\frac{9}{19}.$$Calculate $(1\nabla 2)\nabla 3.$