At this time there is no supported method of specifying only CIFS (SMB) version 3 when mounting a CIFS (SMB) share.
SSB (Syslog-ng Store Box) does not specify a specific version of CIFS (SMB) when mounting. therefore, it adheres to the version set on the endpoint.
If the endpoint is SMBv3 only, then the mount would use version 3.
With this said, the SSB will use any other version should the endpoint use a different version other than SMBv3.
Additionally, when mounting, the SSB will cycle through the following security types, in the following order and will pick the first one that is supported on the endpoint:
'ntlmssp', 'ntlmsspi', 'ntlmv2i', 'ntlmv2', 'ntlmi', 'ntlm'