I dont know about three days, but if you start working, it does come together fast.
First, decide what you want to convey through your statement. Will it be a life story, sequence of events or whatever format you choose. Search online for some samples.
(Brainstorm): Start with putting your thoughts as bullet points, use your resume or life story story to express how and why you began to pursue dentistry (did something or someone have any influence on you, etc.).
Summarize your accomplishments, work, leadership, honors, volunteering; any other activities, clubs or simply whatever makes you "YOU", and how are you different from any other pre-dents.
Brainstorm for a few days, these thoughts dont have to be connected in the beginning. At first it will look like a bunch of randomness, and thats fine. Then, you will have to create some transitional thoughts that will glue that randomness together, this should take another day or two.
After you have an initial draft, start editing and you will get even more thoughts and ideas, that should take no more than a few days as well.
Try to ask your old English professor to help you edit your draft or any English major. I personally used my campus writing services to edit my first draft and then I got my grammar, punctuation and contents checked by my old English professor (he knew me very well).
I roughly spent ten days total (1-3 hours per day).
I am not a native English speaker (started speaking when I was 16), so for most of you guys, one week should be enough.
Always write more than required, as you will remove much of unnecessary wordiness later when you edit.
Hope that helps you in anyway.
Best of luck😀