Will this running plan work?

I plan to run 3 miles in 21 minutes by the end of august. My aim is to run 3 miles in 21 minutes or better for the USMC test. My plan is to start with a mile and bring my time down to 7 minutes and run at this pace for several days. Then start the 2 mile and bring that time down to 14 minutes and finally, bring my 3 mile down to 21 minutes. I know this plan is kind of obscure, but I hope you get the picture. Also, I plan to run three times a week.