I have a timeStamp field in firebase saved in UTC+3 as local time. But when I got it in firebase cloud functions it returns in UTC. How can I not allow firebase to change the timezone when I get this field? I mean if the time is 10 am UTC+3 I don't want to get it 7 am UTC. I need it to stay 10 am. Thanks.
There is no out-of-the-box way for doing what you want.
All timestamps in Firestore are saved in UTC. The timezone of the user who saved the timestamp is not saved with the timestamp value. More details in this SO answer (and comment)
You could add an extra field in the Firestore document with the timezone and do the calculation in your Cloud Function (with a library like moment.js or any alternative) to generate a value according to this time zone.