That's where you're going wrong. 1% of the word count for a well-research essay. 12 would do if it was a 1000 word one, not 3000. At A level it took me much longer to write than at uni. By the second or third year of uni you should be able to look for the key words in the title and quickly work out a basic argument - at 1500 to 2000 words, it's not going to be much more.
If a paragraph is too short, it may be because the initial idea has not been developed sufficiently. To some extent, the level of development is dependent on the writer's purpose and the overall length of the essay. However, you should beware of paragraphs of only two or three sentences. Read them carefully and consider if your idea has been.