Since Activity2 is pushed in the stack the back button of Activity3 will get you to Activity2. Hence the best you can do is not to push Activity2 in the stack.
In your manifest.xml, where you define your activity do the following:
<activity android:name=".Activity2" android:noHistory="true" ... />
developer.android.com says:
A value of "true" means that the activity will not leave a historical trace.
It will not remain in the activity stack for the task,
so the user will not be able to return to it.
For more details:
https://developer.android.com/guide/topics/manifest/activity-element.html
Hope this solves the issue :-) I know this question has been asked long time back, but it might help some...