There is no CA OPS/MVS WebCenter 12.3 software release. The CA OPS/MVS WebCenter 12.2 product release is the only software available for download at this time. There have been no changes to the software. It is compatible with OPS/MVS 12.3.
Download this file from Support.ca.com - Download Center - Product Download page.
CA OPS/MVS 12.2 - WebCenter Package