It means the process exited with a code of 3. There's not much more you can say than that, unless you know what conditions inside the app might cause an exit code of 3, but I do not.
My suggestion would be to run `cf logs` and see if you can get additional information from the application. If not, review how you are deploying the application and check the output of the build pack during staging. If you do not see any logging from your application, it may be failing before it can get a chance to start up, perhaps due to a bad start up command or something that happens during staging.
Hope that helps!
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For reference, there are some standard exit codes but 3 is not one of them.
http://www.tldp.org/LDP/abs/html/exitcodes.html