Basically an impression is the gut feeling you get in the moment when you see someone, or about a situation, you make up all sorts of things in your mind based on your cultural/social upbringing, past experiences, etc that might or might not be true/accurate. As humans, we fill in the blanks all the time with our assumptions.
An assessment is something you make when you have to make a decision about said person or situation. What are you going to do/say based on the data you have? Do you think you can trust? Should you be careful?
An assessment is something that is always up for review as your perception can change and does change as you get to know and understand someone or something better. New information, new perspectives, help you get a more and more accurate understanding. With an assessment you always give people the benefit of the doubt and the chance to explain and to prove you wrong.
A judgment is pretty much final. Just like in court. You have gathered “enough” information to make up your mind and put the person or understanding of a situation “in a box”. A judgment should only be used as last recourse as, although a judgment can be appealed, there is not much hope left and something drastically different would have to come up to consider changing your mind. E.g. a true transformation, revelation, realization.