You need to have single set of KDS (CKDS, PKDS and TKDS) shared among MVS A, B, C.
You have to set same MK (master key) on all systems (LPARs).
However when you change MK on MVS C, you will loose content of CKDS.C, PKDS.C, and TKDS.C
If you don't have any keys inside, that's OK.
Otherwise you have to do the following:
Perform coordinated MK change on MVS C - that include re-encipher of CKDS, PKDS and TKDS.
Now you have same MK in MVS A,B,C. And both KDS sets are available (readable).
But you still have TWO sets of KDS's while you want to have single set.
Well, you have to move the keys from CKDS.C to CKDS.AB. And do the same for PKDS and TKDS.
Assuming you don't have same labels (key names) you can simply copy VSAM records from CKDS.C to CKDS.AB, etc.
HTH