  In this work we investigate the sensitivity to the neutrino mass hierarchy,
the octant of the mixing angle $\theta_{23}$ and the CP phase $\delta_{CP}$ in
the future long-baseline experiments T2HK and DUNE as well as in the
atmospheric neutrino observation at Hyperkamiokande (HK). We show for the first
time that the sensitivity is enhanced greatly if we combine these three
experiments. Our results show that the hierarchy sensitivity of both T2HK and
HK are limited due to the presence of parameter degeneracy. But this degeneracy
is removed when T2HK and HK are added together. With T2HK+HK (DUNE), the
neutrino mass hierarchy can be determined at least at $ 5 \sigma$ (8 $\sigma$)
C.L. for any value of true $\delta_{CP}$. With T2HK+HK+DUNE the significance of
the mass hierarchy increases to almost 15 $\sigma$ for the unfavorable value of
$\delta_{CP}$. For these combined setup, octant can be resolved except
$43.5^\circ < \theta_{23} < 48^\circ$ at $5\sigma$ C.L for both the hierarchies
irrespective of the value of $\delta_{CP}$. The significance of CP violation is
around 10 $\sigma$ C.L. for $\delta_{CP} \sim \pm 90^\circ$. Apart from that
these combined facility has the capability to discover CP violation for at
least $68\%$ fraction of the true $\delta_{CP}$ values at $5 \sigma$ for any
value of true $\theta_{23}$. We also find that, with combination of all these
three, the precision of $\Delta m^2_{{\rm eff}}$, $\sin^2\theta_{23}$ and
$\delta_{CP}$ becomes 0.3%, 2% and 20% respectively. We also clarify how the
octant degeneracy occurs in the HK atmospheric neutrino experiment.
