  We have studied the oxygen-isotope effects on T_{c} and in-plane penetration
depth \lambda_{ab}(0) in an optimally doped 3-layer cuprate
Bi_{1.6}Pb_{0.4}Sr_{2}Ca_{2}Cu_{3}O_{10+y} (T_{c} \sim 107 K). We find a small
oxygen-isotope effect on T_{c} (\alpha_{O} = 0.019), and a substantial effect
on \lambda_{ab} (0) (\Delta \lambda_{ab} (0)/\lambda_{ab} (0) = 2.5\pm0.5%).
The present results along with the previously observed isotope effects in
single-layer and double-layer cuprates indicate that the isotope exponent
\alpha_{O} in optimally doped cuprates is small while the isotope effect on the
in-plane effective supercarrier mass is substantial and nearly independent of
the number of the CuO_{2} layers. A plausible pairing mechanism is proposed to
explain the isotope effects, high-T_{c} superconductivity and tunneling spectra
in a consistent way.
