What you're describing is normal. After porting your WhatsApp number to a different phone, you cannot open WhatsApp on the old one.
Your best option is to move all WhatsApp message to your new phone. The following method will delete all WhatsApp message that you got since switching to the new phone.
In your old phone go to your file manager/internal memory/WhatsApp/database, in that folder find the latest one, and copy it to your new phone in the same place, just remove the ones that are there till now and the only one in the folder should be the backup from your old phone. Then clear data of your WhatsApp app on the s8 (settings/apps/WhatsApp/storage/clear data.) Now open WhatsApp and go through the registration process. When it asks if you want to restore messages, select yes and it will find that backup (if it finds your Google drive backup instead, you should delete that one) now all your messages will be moved to your s8
Another option is to verify a new number on the n3. This will not delete the message. or verify your regular number just for a few minutes and before you transfer back, put your n3 in airplane mode and keep it that way without connecting to WiFi, that way WhatsApp will not know that you ported back.
