ME, BSS and VLR/SGSN are 3G, the ICC is 2G (i.e. a SIM). This applies when e.g. a 2G subscriber with a 3G ME roams in a 3G network.
Any HLR/ AuC will deliver triplets to the 3G VLR/SGSN. The 3G BSS requires CK and IK, so the VLR/SGSN applies conversion function c3 to generate them from Kc. The SIM can only perform 2G AKA and returns SRES, Kc to the ME which also applies c3 to generate CK, IK.
Despite the usage of CK and IK, security is based on Kc, i.e. 2G security context is established.