According to this answer How to solve "Your APK's version code needs to be higher than 2." in Google Play's Developer Console? I have just changed the vers
This seems because you have already uploaded APK file of version 3 to Google Play Store. And again you are uploading apk with same version. So this issue has been occurred.
So for solution you need to change your version name and version code (increment with 1)and run application once and after export it upload to Google Play.