NOT A TEACHER
Hello, Mr. White:
May I add my two bits?
I notice that you identify yourself as an English teacher.
Therefore, I think that it is fair to say that 3,000 words will NOT be enough for you.
As an educated person, you will no doubt be writing to a variety of people, many of whom are also educated.
You do not want to restrict yourself to 3,000 words.
As you read more, you will naturally pick up more words.
At first, those words will become part of your passive vocabulary (words that you understand the meaning of).
Later, some of those words will become part of your active vocabulary (words that you actually use in speaking and writing).
I suspect that you have already learned some new words by reading the posts in this thread.
There are no "big" words or "small" words. There are only words that are the most appropriate.
For example, instead of saying that he eats in expensive restaurants, there are occasions when you may wish to use high-end or upscale.
The bottom line: In my opinion, you should not tie yourself down to a particular number of words. Just keep reading and listening, and your vocabulary will increase naturally. Then your writing will reflect that natural growth.