The PGP Encryption Server is capable of using various ciphers to encrypt email using PGP/MIME or S/MIME. For key generation 3DES is a cipher that cannot be unchecked. The PGP Encryption Server will then generate all keys with 3DES as the default cipher if the preferred cipher is not available to use.
As 3DES is a default fallback cipher, it cannot be unchecked from the user interface of PGP Encryption Server.