The 3 ETH was the gas fees for the transactions. (Some went to deploying the attacking contract, some went to contract interactions afterwards.) With a flash l

The 3 ETH was the gas fees for the transactions. (Some went to deploying the attacking contract, some went to contract interactions afterwards.) With a flash loan, the funds must be returned by the end of the transaction, or the transaction fails. This makes the completion of the transaction the collateral, as if it fails at any point, all transactions (including the loan) get reverted.