For the optional TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 cipher, the certificates must be:
Key: use RSA with 3072 bits or more
Signature algorithm: sha384WithRSAEncryption
The cipher rules apply to all certificates starting from the CA certificate, any intermediate certificates (if used) and up to the client and server certificates.
This means that 192-bit mode does not allow weaker CA certificates to sign stronger server/client certificates or mixing RSA CA certificates to sign EC certificates.