How can this Encryption Class be more secure?
Some issues:


No MAC => say hello to padding oracles. This is fatal flaw that allows decryption of the message in many usage scenarios (when there is an active attacker).
You use CBC mode with constant IV. A new IV must be chosen for each message. It should be unpredictable and random.
Your keying is only secure with really strong passwords (over 100 bits of entropy). I recommend using an actual binary key drawn from a secure CSPRNG encoded with Base64 instead of a password.

A related stylistic problem is that you don't clearly distinguish between keys (which should be uniformly random) and passwords.
Does the key ever leave the trusted systems? It looks like you're hardcoding a key into a client, which might run on an untrusted machine. Even if it stays on the server, put it in a config file, not the code.

Your program should be secure even if the whole code is known. Logistically it's also much easier to keep a config file secure, since it doesn't need to be checked into version control.
Why would you choose 3DES over AES in new code? AES is faster and stronger. It also has 128 bit blocks instead of 64 bit blocks, so it doesn't weaken once you encrypt a couple of gigabytes using the same key.
All those streams are unnecessarily complicated. Simply call encryptor.TransformFinalBlock on a byte array instead.