Log into APIC 3 and run the command "show cores" to see if any processes have recently crashed. Could you also provide output for "acidiag avread" and "acidiag fnvread"?
Are all your APICs running the same firmware version? This can be seen by running the command show version on the APIC. Is your APIC connected to leaf1? You can see this by using the show lldp neighbors on leaf 1 and see if APIC 3 shows up under the device IDs and are connected to the correct local interface and port ID. Also, if you do not have access to APIC 3 you can use the acidiag avread on the leaf as well. As Ben stated, if you can provide this output we can verify if your cluster size, appliance ID, TEP addresses, firmware version, and other things. It is possible the leaf has a stale config for APIC3 if it has previously joined the fabric.
You can try:
acidiag avread and look for health and chassis id.
ping TEP address of other APIC's and make sure u are able to ping.
I understood APIC3 is unavailable and If you want to restore APIC3 to fully fit, try this:
Log into APIC1 and decommission APIC3 from system > controller > APIC1 > cluster as seen by node > right click on apic3 and decommission. (See the screenshot)
Go to APIC3
acidiag touch clean
Make sure you cant login to the APIC with creds. This means it does not have any config.
This basically brings the APIC with clean configuration.
Go back to APIC1 and commission the old one. Old one state would be un-registered. Once its commissioned it ll be fully fit.