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Actin nucleation is one of the key control points in cellular regulation of actin cytoskeleton dynamics. The Arp2/3 complex is an evolutionarily conserved actin nucleator that binds to the side of an existing actin filament and polymerizes a new daughter branch. We used single particle electron microscopy to compare the structures of Arp2/3 complexes bound by inhibitory ligands: GMF, Coronin, and Arpin. Each inhibitor appears to have a distinct binding site on Arp2/3 complex, yet they each cause the complex to adopt ‘open’ nucleation-inactive conformations. Binding of GMF induced two distinct and novel forms of the open conformation of Arp2/3 complex, possibly consistent with it having separate binding sites on Arp2 and Arp3. Binding of Arpin induced the standard open conformation of Arp2/3 complex, and tagging Arpin revealed that it may also bind near or on Arp2 and Arp3, consistent with its competitive interactions with VCA for binding Arp2/3 complex. Additionally, we identified that Arp2/3 activator Abp1 may bind near Arp3 and induce the 'closed' primed for nucleation conformation of the complex. Overall, these results reveal similarities and differences in the mechanisms of the three inhibitors. Coronin and Arpin both induce a similar open/inactive conformation; yet have highly distinct binding sites on Arp2/3 complex. In contrast, while GMF and Arpin have neighboring binding sites on Arp2/3 complex, and both compete for binding with VCA, they induce distinct inactive conformations, pointing to differences in their functions.