Xcode, the iOS SDK and the Simulator come bundled together in one installer because they share libraries and other stuff in the Developer folder. Mismatched combinations probably won't work. Even if you somehow manage to install the 3.0 Simulator with your current Xcode+SDK installation, it would probably crash.
So you need to find a complete installer for an old iPhone OS 3.0 SDK with Xcode 3.x, and install with that on a Mac that to get a working iPhone OS 3.0 Simulator. You may or may not be able to get an SDK/Xcode combination that old that runs on OS X 10.6 Snow Leopard, so you may need to find an older Mac that can boot into OS X 10.5 Leopard as well.
Added:
Anyway, a developer should only be supporting 3.0 devices by actually having one around on which to test their app, before setting the Deployment Target that low when submitting an app for live customers use. Customers are not supposed to be some test dummies for random developers.
Just test apps on the oldest, lowest OS version, device you can find (beg, borrow, buy used and dented, drive across town), and use that as your Deployment Target. If you can't find a device that old, neither can the vast majority of the customers for your app.
- Download xcode_3.2.6_and_ios_sdk_4.3.dmg
(note - you need a developer account to access this software. I have instead linked to an open-to-the-public Apple discussion that contains the direct download link
- Mount the .dmg file
- Inside the disk image there is an invisible "Packages" folder. To open it select "Go to folder..." command from Finder menu bar and digit the path "/Volumes/Xcode and iOS SDK/Packages/"
- Now double-click "iPhoneSimulatorSDK4_2.pkg"
- Click into the destination disk and a button "Select folder..." will be revealed
- Click the above button and select your "Developer" folder
- Install the package, and Restart Xcode.
This will install the Simulator 4.2 SDK (both iPad and iPhone) inside the folder "/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/"
Now inside XCode 4.2 you can select this simulator from the menu.
Bye.
Best Answer
Officially, no. You can't install appstore apps on the simulator, your best shot would be to talk with SwiftKey and see if they give you access to the source code, this way you could compile it to the simulator and activate the keyboard, but I really doubt they would do that.