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.
