  We have investigated the optimally doped tri-layer cuprate
Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ (Bi2223) by angle-resolved
photoemission spectroscopy, and observed energy bands and Fermi surfaces
originated from the outer and inner CuO$_2$ planes (OP and IP) separately. The
OP band is overdoped (hole density $\sim$ 0.26/Cu) with a large d-wave gap of
$\Delta_0$ $\sim$ 43 meV while the IP is underdoped ($\sim$ 0.06/Cu) with an
even large gap of $\Delta_0$ $\sim$ 60 meV. We propose that the enhancement of
the $\Delta_0$ of IP is due to the minimal influence of out-of-plane disorder
and that the OP gap is then enhanced through interlayer coupling, most likely a
proximity effect from the IP.
