https://wccftech.com/galaxy-z-fold-3-will-not-have-a-dedicated-s-pen-slot/

If the rumors are true, then we will see Samsung release the Galaxy Z Fold 3 in July, and the good thing is that this time around, we are getting to see another device with the revered support for S-Pen. However, up until now, we were not sure if Samsung would include an S-Pen housing in the phone or it will be sold or shipped separately just like it does with the Galaxy S21 Ultra. We have another report that suggests that Samsung will not have an integrated housing for the S-Pen, which means that you will have to buy it separately but considering the higher price of the foldable, we are hoping that Samsung includes the S-Pen.

Based on a report coming from NaverNews, Samsung actually had planned on including an S-Pen slot on the Galaxy Z Fold 3, similar to what we see on the Galaxy Note series. Still, the company had to go ahead and ditch the slot altogether because it would have compromised the device's dust and water resistance.