I believe that all the healthy ones that will hatch, often do so within approx 48 hours of the first hatch. After the end of day 23, the chances of healthy hatches goes down considerably. Of course, as soon as I say that, others will say "I had a chick hatch on day 25 and it was fine!" But I think you have to look at the whole picture.
Here's how I look at it: You have had 3 hatch already on day 21, and have several more in progress. This shows your temperature and humidity were good for a "normal" 21-day hatch. (If your temps were low, your whole hatch could have been delayed.) I'd give them through Monday, then re-assess the remaining eggs for signs of life.
Here's how I look at it: You have had 3 hatch already on day 21, and have several more in progress. This shows your temperature and humidity were good for a "normal" 21-day hatch. (If your temps were low, your whole hatch could have been delayed.) I'd give them through Monday, then re-assess the remaining eggs for signs of life.