Twenty-one two-proton knockout (p, 3p) cross sections were measured from neutron-rich nuclei at similar to 250 MeV/nucleon in inverse kinematics. The angular distribution of the three emitted protons was determined for the first time, demonstrating that the (p, 3p) kinematics are consistent with two sequential proton-proton collisions within the projectile nucleus. Ratios of (p, 3p) over (p, 2p) inclusive cross sections follow the trend of other many-nucleon removal reactions, further reinforcing the sequential nature of (p, 3p) in neutron-rich nuclei.