bash-3.2$ export SECUDIR=/usr/sap/<SID>/<Instance>/sec
bash-3.2$ sapgenpse import_own_cert -p <PSENAME> -c /tmp/cert.crt -r /tmp/inter.crt -r /tmp/root.crt
import_own_cert: Installation of certificate failed
ERROR in ssf_install_CA_response: (1280/0x0500) Incomplete FCPath, need certificate of CA : "CN=Symantec Class 3 Secure Server CA - G4, OU=Symantec Trust Network, O=Symantec Corporation, C=US"
ERROR in ssf_install_certs_into_pse: (1280/0x0500) Incomplete FCPath, need certificate of CA : "CN=Symantec Class 3 Secure Server CA - G4, OU=Symantec Trust Network, O=Symantec Corporation, C=US"
Based on your advice, I think I tried it correctly.
Here is where I downloaded Verisign's Root and Intermediate certs: