When the Lead is converted to Account, Contact and Opportunity in Salesforce, ConvertedContactId on the Lead Object will contain the Contact Id after the Lead Conversion.
Sample Query:
SELECT Name, ConvertedContactId
SELECT Name, ConvertedContactId
FROM Lead
WHERE ConvertedContactId != null
Similarly, ConvertedAccountId and ConvertedOpportunityId will contain the Account Id and Opportunity Id after the Lead Conversion process.
Similarly, ConvertedAccountId and ConvertedOpportunityId will contain the Account Id and Opportunity Id after the Lead Conversion process.