Lmp was 17th jan, positive hpt 16th feb, clearblu dig: 2-3 weeks on 18th feb. 10th march (7wks 3ds)scan showed yolk sack but not fetus. viable?

Likely: If your doctor saw a yolk sac, that is a very good sign. A yoke sac means likely there is a viable pregnancy there. Having said that, of course, a yolks sac at this point does not correspond to the dates that you gave. It's very possible that you are pregnant with a viable pregnancy but the pregnancy is much younger than the seven weeks you've calculated. Ask for another ultrasound in a week.