The Tab S3 has no dedicated slot for storing the S-Pen (unlike previous tabs equipped with the same tech) and the S line is not aimed squarely at content creators, which could be why Samsung opted to not include one in the sales package.
All S-Pens are backed by Wacom tech - the battery-free pens and their precise tips are a signature of Wacom's EMR (Electromagnetic Resonance) technology. Meanwhile, most if not all modern phones have a capacitive touch screen. You have to explicitly look for "S-Pen" when you need a device with it.
As of now (Sep. 2017), most S-Pen-equipped Samsung tablets still start their model numbers with a common letter "P":
- Galaxy Note 10.1 2014 is P60x (discontinued)
- Note 12.2 is P90x (discontinued)
- Tab A 2015 with S-Pen is P35x (8.0-inch) and P55x (9.7-inch)
- Tab A 2016 with S-Pen is P58x (10.1-inch) — (discontinued)
(x for variant, e.g. 0 for WiFi, 5 for international LTE)
Tab S3 is a rare exception but it's marketed heavily around its S-Pen so you'd still know it easily.
Samsung once had a "C-Pen" denotation for a pen used on pure capacitive screens, although that's long obsolete.