What's the equivalent of Smart Change Detection in version 3?

The old Smart Change Detection feature in version 2 has been replaced by the more flexible Filter feature in version 3, but it still supports the same change detection functionality, plus much more.

You can read about the Filters feature in the help book.

The replacement for the old Smart Change Detection feature is the Block filter. It works the same way, with Start and End text blocks, but has some extra options for more flexibility (hidden away till you need them). Your version 2 tests will have been automatically upgraded to use the Block filter if you specified Start and End text, or the Change filter if you didn't. (The Change filter is actually based on the Block filter plug-in, but omits the Start and End text fields, so it just looks for any change.)

Read about the Block filters in the help book.