Can you please elaborate your issue,
And add some screenshots so we can understand your issue,
The field that you wish to change is an attribute of an object. In order for a value calculated in a microflow to be reflected on the page you will need to change the value of the given attribute of the context object on the page.
This can be done with a change object activity in your microflow. If you wish the calculated value to be a permanent change to the object then you should select the ‘commit object’ and ‘refresh on page’ radio buttons.
Here is an example of how to change the name of a filedocument in such a way that if the field is an the page the user is on it will be refreshed and committed to the database.
Hope this helps
What you need to do is fire a Microflow (or Nanoflow) on the OnChange event of your dependent dropdowns. This microflow will take the object in question as a parameter and then you can change the object in that microflow and refresh it in the client.