Given that our crystal balls are not connected to your app, you are in a much better position to hunt down this bug rather than everybody else hypothesizing wildly ;-)
If you can isolate the problem down the section or line of code where the wheels are falling off, then you will be 87.3% of the way to the solution already.
What does the log say when you run it in debug mode?
What code gets executed when you "press the button to reopen activity3"?
Add trace Logs to the start and end of activity3's Activity_Create and Activity_Resume Subs. Do they show up in the log? If you know that the problem is occurring between points B and F in your code, then you can add further logs to divide and conquer, to narrow in on whereabouts the problem is occurring and then to determine what is causing it to occur.