I need to decrypt a column in a table that has previously been encrypted at application level.
The algorithm is DES at 192 bits and block size = 64.
I have the password but DecryptByPassPhrase doesn't seem to work.
You need the original key. Most likely the encryption password was used to encrypt the key, not the data.
If the encryption key was encrypted by a certificate and the certificate was encrypted by a password (As is often the case) then you obviously need open the certificate using the password. Whoever designed the original encryption scheme should be able to guide you through the proper decryption key hierarchy.