Creating multiple independent keys for cascading - recommended approach?
$\newcommand{\concat}{\mathbin\Vert}$Use a key derivation function, such as HKDF, to expand a master password-derived key into subkeys for each cipher.


Compute $k = \operatorname{scrypt}(\mathit{password}, \dots)$.
Compute $k_\mathrm{AES} = \operatorname{HKDF}_k(\text{‘AES’})$.
Compute $k_\mathrm{Serpent} = \operatorname{HKDF}_k(\text{‘Serpent’})$.
Compute $k_\mathrm{Twofish} = \operatorname{HKDF}_k(\text{‘Twofish’})$.
Throw out Applied Cryptography and get Cryptography Engineering to replace it, but make sure to print out the unauthorized errata and glue them into your copy of it.
Step back and write down a clear security model for your application, because higher-level logic in your application will be attacked long before the crypto.