In each case, it's the context that determines the most suitable tense.
I have been working at ....
means, as you state, that you are still working at.... It stretches from a point in the past and continues into the present.
I have worked at.....
is the response that you would give, for example, if you were asked what experience you had in your field. Because this experience covers the period up to the present day, you might respond:
As well as obtaining a PhD in software development, I have worked at Google (for ten years) and have completed two years with Microsoft.
This tense stretches from a point in the past up to the present, without the implication that it is continuing.
I worked at.....
is a statement of something you did at some undefined time in the past and which you no longer do. You would use the past tense if you were asked, for example, where you had worked after leaving university. You might reply:
I worked at Google for ten years (before I went freelance).