At 3 weeks, I would think the slow flow would still be appropriate. I used the playtex nursers, and ds was a couple of months old when we switched to the next flow.
BUT - every baby is different. You could always buy one pack of the medium flow, and try it. The concern is that you don't want to choke/flood them with milk, but if your lo does ok with it, go for it!
If not, you're not out anything, as you'll have to buy them eventually! :wink: