Ionization of the 3p orbital of argon (Ar) monomers, dimers (Ar2), and small-size Ar clusters (Arn,(n)≈15) is investigated for 90-eV electron impact. Experimentally, the three-dimensional momentum vectors of the two outgoing electrons and the residual ion are measured in triple coincidence, as is the mass-over-charge ratio of the ion, using a reaction microscope that covers a large part of the final-state phase space. The triple-differential cross sections (TDCS) are obtained for almost the full solid angle of the ejected electron, in which the data for Ar monomers are well reproduced by B-spline R-matrix calculations for ejected electron energies of 3 eV, 5 eV, and 10 eV, and projectile scattering angles of 10∘ and 20∘. Compared to the Ar monomer, the cross sections for Ar2 and Arn exhibit effects due to the neighboring atoms by the suppression of binary and recoil lobes and the enhanced electron emission out of the projectile scattering plane. These environmental effects are further investigated using a multicenter three-distorted-wave theory, which can generate the TDCSs for electron-impact ionization of Ar2. These studies reveal an important role of multicenter scattering reactions on the ionization dynamics of Ar clusters. Moreover, a two-center interference is not found in the TDCS of Ar2. This is interpreted as due to the involvement of orbitals with both gerade and ungerade symmetries in the ionization process.