Answer
$$ R_{uvw}= 2(v+w)^{-3}=\frac{2}{(v+w)^{3}}.$$
Work Step by Step
Since $ R(u,v,w)=\frac{u}{v+w}$, then we have
$$ R_{u}=\frac{1}{v+w}=(v+w)^{-1},$$
$$ R_{uv}= -(v+w)^{-2},$$
$$ R_{uvw}= 2(v+w)^{-3}=\frac{2}{(v+w)^{3}}.$$
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