When did Sakamoto get his three chips?
Sakamoto has 3 chips, plus his own (for a total of four), after he kills Masashi Miyamoto (knife guy) and Souichi Natsume (attorney guy with glasses). 

Before Miyamoto gets killed (while he has Himiko hostage, after Natsume is killed by the Cracker BIM), he mentions he's already killed a guy and taken his IC chip and BIM. Miyamoto wrongly assumes that Sakamoto has three chips (two plus his own) since he has 3 types of BIMs, and that Himiko has Mitsuo Akechi's (fat guy) IC chip.