The burndown charts use the "estimation statistic" selected in the board configuration. That statistic must be a simple number (you can't do the required arithmetic on non-numbers), so the selection you are seeing are the three purely numeric fields you have in your system.
Time estimates are one field that's always there, Story points is the second that I would expect (Jira Software creates it by default, but it can be removed or renamed), and your third one is some other numeric custom field your admins have added.
If you want to see and use other fields there, then get your admins to add custom fields of the type "number"
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