This question is badly lacking from context. Vost - can you please provide some (some sample sentences at least), and can all responders await the extra context before replying so that the answers are directed and appropriate. Thank you for your understanding.
Thank you - that helps direct the comments. In this context I think you can use all three with very little difference in meaning. I think that "am under the impression" is least appropriate because it implies a permanent state (or at least one of some duration) that is at odds with an impression you have only from time to time. Of the other two the "get" is more passive since the impression is given to you - and I think that would be best.