We study two- and three-gluon glueballs of $C=+$ using the method of QCD sum rules. We systematically construct their interpolating currents, and find that all the spin-1 currents of $C=+$ vanish. This suggests that the ground-state spin-1 glueballs of $C=+$ do not exist within the relativistic framework. We calculate masses of the two-gluon glueballs with $J^{PC} = 0^{pm+}/2^{pm+}$ and the three-gluon glueballs with $J^{PC} = 0^{pm+}/2^{pm+}$. We propose to search for the $J^{PC} = 0^{-+}/2^{-pm}/3^{pm-}$ three-gluon glueballs in their three-meson decay channels in future BESIII, GlueX, LHC, and PANDA experiments.
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