The third system IPL

Figure 1 shows the multisystem environment after SYSC is initialized.

Figure 1. Three Systems in the Multisystem Sysplex

When SYSC is initialized and joins the sysplex and the global resource serialization complex:

XCF coupling services are available on SYSA, SYSB, and SYSC, including signaling services between the three systems.
The same primary sysplex couple data set (not shown) is shared by SYSA, SYSB, and SYSC.
MVS™ component exploiters (such as global resource serialization) use signaling services between SYSA, SYSB, and SYSC but must use non-XCF connections to communicate with SYSD.
Multisystem applications can exploit XCF coupling services and signaling services on SYSA, SYSB, and SYSC.
SYSA, SYSB, and SYSC must share a Sysplex Timer if they are running on separate CPCs (not shown).

Consider this mixed complex a temporary configuration during migration until the sysplex matches the complex.