I know that the Fitbit Charge 3 has a weather app, but according to the Fitbit community, you need to turn on all day sync for the weather app. My suggestion is to ask that Fitbit make accessing the weather not require your phone with you all day. For example, searching "weather" on google will give you a panel showing the current weather, and also the predicted temperatures. Can a Fitbit just get all the predicted temperatures in one sync with your phone? Then Charge 3 can just save the temperatures and display them according to the time.
Moderator Edit: Clarified subject and updated label
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