Sure, you could select the Rounded Rectangle tool icon and Option + click to bring up a dialog box allowing you to specify the radius of the corners. But again, once the rectangle is dragged out, that’s it. There’s no going back and changing that radius later, and attempts to resize the rectangle later on actually resizes or stretches the rounded corners.
If you want to keep full control over those rounded corners of your rectangle, use the normal squared-edge Rectangle tool icon in the Tools panel. Once your rectangle is dragged out on the page, go to Effect>Stylize>Round Corners… in the menubar.
In the dialog box that appears, you can specify a custom corner radius amount. And here’s the great part. This is an effect applied to the rectangle, not the actual rectangle path.
So now when you stretch that original three inch wide rectangle to nine inches wide, the corner radius doesn’t stretch with it, it stays exactly at the amount you specified. As an added bonus, you can go back later and change that radius amount if you wish via a quick visit to the Appearance panel.