As far as I know, 'to be interested in something" is the only right option, You can check it out in your english dictionary I'm not quite sure. Let's wait for a better answer.
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Nona The Brit'to be interested in' and 'to be interested about' have the same meaning.Hi NTB
To be interested at doesn't have the same meaning. It can be used in some contexts but the 'at' would not apply to what you were interested in itself. For example. I'm not interested now but I might be interested at a later date.