
Explanation
Volume C on = Computer 2 and 3 only
Volume D on = C1, C2, C3
The "Require additional authentication at startup" setting is located in the Computer Configuration\ Administrative Templates\ Windows Components\ BitLocker Drive Encryption\ Operating System Drives path. If the policy is disabled and there is no TPM chip installed, operating system drives can not be encrypted using BitLocker. Additional data volumes can be protected using a password as a key protector for example.
References:
https://www.windowscentral.com/how-use-bitlocker-encryption-windows-10