I'm not an experts on the couch to 5K, but if you can jog 1.5 miles, you cna just add a little bit each time in a way that makes sense to you. A general rule of thumb is to increase your mileage no more than 10% a week. So if you are running 1.5 miles 3 days a week, that's 4.5 miles a week, so next week you could add .45 mile somewhere! You could add a little bit to each current run, or you could take the total mileage of 4.95 (yeesh, I'll just round it to 5) and break it into 4 runs. But I would say, if you can run 1.5, you can run 2 miles. So just increase it a little at a time each week and you'll be at 5K before you know it. Go slow with the increases, stretch well, hydrate, and you do have good shoes, right?
"And that's how Beowulf rolls"~ my DD