"Keep doing just three miles up until the 5K"

Yes you can do that. Or you can increase your mileage by adding one day a week of a somewhat longer run. Say Mon. Wed. you run 3-miles and Sun. you run 3.5-4miles or increase as you feel comfortable.

If I were you I would keep running 3-miles in each of my running sessions (I only do 3 days a week). And then after the race, start towards a new goal. I chose a 10K plan and now I am on to the Half-Marathon, for example. Or you could train for something fun and more specific like a long distance relay -- the Ragnar races or the Odyssey races (Tucson or Tahoe) come to mind....

Have fun!

I completed W9D1 this morning. A group of us have just finished or are close to finishing and most of us plan to move on to the One Hour Runner program. Come join us - we are the mile loggers in the Newbie cafe.