No, in order to be convicted, a Judge or Jury must find you guilty. If your case was dismissed then, there was no case for a Judge or Jury to hear and therefore you have not be found guilty of anything because there is no case.
You can answer no since a Nolle Prosse is a dismissal.
You are probably eligible to have those arrests "restricted" from your criminal history, which would prevent most employers from seeing the arrests.
Please let me know if you have any more questions.
If your charges were dismissed, then you were not convicted. If they ask if you have been arrested, then the honest answer would be yes, but if they are asking about convictions, then no.