Answer
$\log(xy^{3})$
Work Step by Step
$\log x+3\log y=\quad $...apply the Power Rule: $\quad \log_{b}(M^{p})=p\cdot\log_{b}\mathrm{M}$
$=\log x+\log y^{3}\quad $...apply the Product Rule: $\quad \log_{b}(MN)=\log_{b}\mathrm{M}+\log_{b}\mathrm{N}$
$=\log(xy^{3})$
