  The quantum spin liquid (QSL) is an exotic phase of magnetic materials where
the spins continue to fluctuate without any symmetry breaking down to zero
temperature. Among the handful reports of QSL with spin $S\ge$1, examples with
magnetic ions on a three-dimensional magnetic lattice are extremely rare since
both larger spin and higher dimension tend to suppress quantum fluctuations. In
this work, we offer a new strategy to achieve 3-D QSL with high spin by
utilizing two types of transition metal ions, both are magnetically active but
located at crystallographically inequivalent positions. We design a 3-D
magnetic system Ba$_3$NiIr$_2$O$_9$ consisting of interconnected corner shared
NiO$_6$ octahedra and face shared Ir$_2$O$_9$ dimer, both having triangular
arrangements in \textit{a-b} plane. X-ray absorption spectroscopy measurements
confirm the presence of Ni$^{2+}$ ($S$=1). Our detailed thermodynamic and
magnetic measurements reveal that this compound is a realization of gapless QSL
state down to at least 100 mK. Ab-initio calculations find a strong magnetic
exchange between Ir and Ni sublattices and in-plane antiferromagnetic coupling
between the dimers, resulting in dynamically fluctuating magnetic moments on
both the Ir and Ni sublattice.
