Welcome to the Forums @Nav1234.
At this moment the OnePlus 3T is not part of the supported devices list. This means that the app has not been fully optimized to work with that phone.
As long as it fulfills the minimum requirements, Android 4.4 and Bluetooth 4.0, there is a possibility that it can work but we cannot guarantee it. In case it doesn't, you could still use other phones or a computer to sync your Fitbit and use the 3T to check your stats on the app.
Hope this answers your question, let me know if there is anything else that I can help you with.
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