  Results are presented for the transverse single-spin asymmetries of direct
photons, neutral pions, and eta mesons for $|\eta|<0.35$ from $p^\uparrow + p$
collisions with $\sqrt{s} = 200$ GeV at PHENIX. As hadrons, $\pi^0$ and $\eta$
mesons are sensitive to both initial- and final-state effects and at
midrapidity probe the dynamics of gluons along with a mix of quark flavors.
Because direct photon production does not include hadronization, the direct
photon TSSA is only sensitive to initial-state effects and at midrapidity
provides a clean probe of the gluon dynamics in transversely polarized protons.
All three of these results will help constrain the collinear twist-3 trigluon
correlation function as well as the gluon Sivers function, improving our
knowledge of spin-dependent gluon dynamics in QCD.
