Why the Cost Explorer shows data for the last three months only

This usually happens if an account leaves Organization and joins new one (accepts invitation from a new Payer). When account is moved to a different Payer's Organization, the history around costs in Cost Explorer prior to joining that organization is not visible anymore. When logging into that linked account, you could still see the charges for previous months in the Billing Console (Bills page of the billing console), and you'd be able to see which Payer paid for which charges. However, with Cost Explorer reporting it's different, and when switching Payers, you would not be able to see the charges prior to switching (and you can't backdate the report or populate it with the data from the previous Payer...), and this information would not be exposed to the new Payer either.