I currently have an Android project in Eclipse.
I created it with a target build of 1.5 (sdk 3).
Now I want to change it so that it has a minSdk of 3 and tar
You can change your the Build Target for your project at any time:
Right-click the project in the Package Explorer, select Properties, select Android and then check the desired Project Target.
Edit the following elements in the AndroidManifest.xml file (it is in your project root directory)
In this case, that will be:
Save it
Rebuild your project.
Click the Project on the menu bar, select Clean...
Now, run the project again.
Right Click Project name, move on Run as, and select Android Application
By the way, reviewing Managing Projects from Eclipse with ADT will be helpful. Especially the part called Creating an Android Project.