  The isospin asymmetries of $B \to K\mu^+\mu^-$ and $B \to K^{*}\mu^+\mu^-$
decays and the partial branching fractions of the $B^0 \to K^0\mu^+\mu^-$, $B^+
\to K^+\mu^+\mu^-$ and $B^+ \to K^{*+}\mu^+\mu^-$ decays are measured as
functions of the dimuon mass squared, $q^2$. The data used correspond to an
integrated luminosity of 3$~$fb$^{-1}$ from proton-proton collisions collected
with the LHCb detector at centre-of-mass energies of 7$\,$TeV and 8$\,$TeV in
2011 and 2012, respectively. The isospin asymmetries are both consistent with
the Standard Model expectations. The three measured branching fractions, while
individually consistent, all favour lower values than their respective Standard
Model predictions.
