The vSphere Client 6.0 (the traditional Windows-based application) is no longer actively hosted on primary VMware/Broadcom landing pages as it has reached End of General Support. However, it can still be acquired through specific repository links or directly from your ESXi host. Direct Download Links
You likely need the vSphere Client 6.0 if: vsphere client 6.0 download link
What to look for: Search for the filename VMware-viclient-all-6.0.0-5112508.exe (this is the version for Update 3). The vSphere Client 6
Installing vSphere Client 6.0
VMware-viclient-all-6.0.0-xxxxxx.exe