Couch to 5k in 3 weeks?

Not possible, you can still enter but plan on coming in beyond dead last. lol
All you can do is start walking several miles a day interspersed with short bursts of jogging/running, for a couple hundred yard spurts. 3 weeks is not, even remotely, enough time to get in marathon shape. More like a year.

You are also at high risk for injury going from couch to track, shin splints, strains, sprains. Don’t forget to stretch and do some warm ups before you hit the road.

I am guessing no, but you can probably go from couch to O.R. in that amount of time if you overdo the training.

To begin, 5 kilometers = 3.1 miles, not 4.61 miles (which would be about 7.4 kilometers). Anyway, three weeks gives you 21 days. I suggest using the Mountie method. Once you figure out what your actual distance goal is (5 kilometers/3 miles or 7.4 kilometers/4.61 miles), divide that by 21.

Let’s say you’re goal is actually 5 kilometers. That means you need to gain about ¼ of a kilometer per day to reach your goal on time. So on Day 1, walk/jog for ¼ km. Then on Day 2, run the first ¼ km and then walk/jog the second ¼ km. Keep adding ¼ km to the running portion each day, finishing with ¼ at a walking or jogging pace. Then on the day of the marathon, you can run the whole way.

If this feels too easy, feel free to increase the added distance to ½ of a kilometer instead of ¼. Never feel bad about running more, but definitely don’t let yourself do less than the minimum. And of course, don’t forget to warm up before you run and stretch after you run. Do not do any kind of static stretching before you run, though. Just do warm ups and dynamic stretches. You can find a pretty helpful guide for warming up here.

Good luck!

And possibly most important of all, hydrate!

It depends upon how old are you and what kind of shape are you in now. Can you walk 4 miles right now? (I’d bet if you are under 35 and are no more than 25 pounds overweight you can do it.)
If you are in reasonable shape and don’t care about the time it takes you, you can do the 5k! Couch to 5k has you walk for a bit (say 5 minutes) and run for a bit (say 30 seconds). You do that a couple of times and then gradually increase the running portion. % and 1, 5 and 2 etc.
Even if you run half and walk half and come in last place you are still WAAAY ahead of the people sitting on the couch watching junk on TV.
Do it! Accept the challenge! You will be proud you did!
Be sure to get the T-shirt!

I did a couple of 5Ks when I was running, and lots of people finished at just a hair over walking speed.

Do your best!