Tier 3 (COMSEC) definition in Computer Security terms:
Definition(s): The lowest tier or layer of electronic key management system (EKMS) architecture comprising hand-receipt holders who use an electronic fill device (e.g., the Data Transfer Device (DTD), Secure DTD2000 System (SDS), Simple Key Loader (SKL)) and all other means to issue key to End Cryptographic Units (ECUs). Tier 3 elements receive keying material from Tier 2 activities by means of electronic fill devices or in canisters (for physical keying material).
Source(s): CNSSI 4009-2015 (CNSSI 4005)
reference: CSRC Glossary