I recently bought VB3 after using VB6 for a few years. Here's a few things I've noticed, bearing in mind that you're going up in version.
1) Check out the Private, Public and Global keywords for variables, constants, subs and functions. They've changed a bit in what you can and can't do.
2) VB6 has two neat "new" variable types: Byte (which is great for working with binary data) and Boolean (something is either true or false; you can use the Not keyword to change the variable's value easily).
3) VB6 lets you create your own classes, kind of like smooshing Modules, Functions and Types into one thing.
4) Predefined constants without copying from constants.txt. For example, VB6 knows that vbYes is what you get from clicking the Yes button on a message box.
5) VB6 provides relief from some of VB3's headaches while providing some state-of-the-art reasons to bang your head against a wall.