I would like to be able to fit 3 barstools. How long does the bar need to be in order to accommodate 3 people comfortably?

The NKBA recommends 24” per seated dinner. Therefore, your answer is 72”. If the stools have arm rests add about 3” per stool. That would make it 81” long. Remember, these are recommendations and could be reduced but it will not be comfortable for dining for long periods. Since you are using stools I assume the counter height will be either 36” or 42”. The knee space recommended for 36” is 15” and 12” for the 42” counter.