Sorry, but with only a few days, there isn't much advice we can give that will help you. Also, there isn't much you can do. Also, three days isn't enough time for training to have an effect.
You have to walk because you are going too fast. Slow down to a pace you can hold for 3 miles. Unfortunately, there isn't enough time for you to find out what that is. You should rest the 2 days before your time trial. So, you have to guess what pace to run.
How did you get into this situation? Have you been training regularly? If yes, for how long? If you have been training for some time, and you were able to do 3 miles without slowing to a walk before, at the same effort you are trying now, what happened? If you don't know, you may be ill. You should tell your family doctor about this.
But, if you haven't been training, what you are experiencing is typical of new runners.
Some people get better times by walking. The rest allows them to go fast enough between walks to more than make up for time lost due to walking.