SNMPv3 credential sets are associated with nodes via an entry in Orion.NodeSettings with SettingName='ROSNMPCredentialID' (for credentials used in read operations) and SettingName='RWSNMPCredentialID' (for credentials used in write operations).
There are two ways to add an SNMPv3 node in the API: by creating an instance of Orion.Nodes or by running a discovery. They each have strengths and weaknesses, but in general I would recommend using the discovery method. And for what you are asking about - specifying SNMPv3 credentials by referencing a pre-existing credential set instead of providing all of the details each time - only the discovery method supports it.
See this example script: https://github.com/solarwinds/OrionSDK/blob/master/Samples/PowerShell/DiscoverSnmpV3Node.ps1