How to get access to the private key for an account when creating a private network using ./sandbox up?

Hi,

I am able to deploy my smart contract and send some data to the Algorand testnet blockchain. Based on my understanding, testnet blockchain is a public blockchain but I need to test on the private Algorand blockchain. When I run the private Algorand blockchain using Sandbox, it will start with three pre-loaded accounts. I can see the account addresses but in order to create my app on the blockchain, I need to have the private key as well. Could you please let me know how I can get the private keys for the three pre-loaded accounts in the Algorand private network?

Thank you in advance for your time,
Sahand

Thank you very much for your response. That answers my question.