Event Envelopes are only available in Cubase Artist or higher.
In your case, I would recommend buying the upgrade from Cubase SX 3 to Cubase Pro 9.5 (it’s one of the last options in the shop page). It will be only a bit more expensive than Cubase Artist, and it comes with many more features, specially for people recording and editing audio.
Cubase 9.5 is a huge upgrade over Cubase SX 3. These pages show a few of the features present in the current versions of Cubase if you’re curious: