The newer processor makes the Apple Watch SE more relevant in 2020 and the future. For example, the Series 3 doesn’t support Apple’s new Family Setup plan that lets you set up an Apple Watch for your child, with parental controls for messaging and location tracking. The Apple Watch SE does.
Then there’s the display and body. The Watch SE has a Retina display that’s 30% larger than the Series 3. That said, neither has the always-on display found on the Series 6. The SE comes in silver, space gray, and gold, whereas the Series 3 is available in just silver and space gray.
The SE also supports features like fall detection and noise monitoring and comes in cellular and noncellular models. Apple only sells the Series 3 in a non-cellular version. However, you might be able to find the Series 3 cellular version through other retailers.
The SE also has a compass, an always-on altimeter (the Series 3 has an altimeter, but it’s not always on), a second-generation optical sensor (the Series 3 has a first-generation one), and a W3 chip for connecting accessories like AirPods. The Series 3 has a W2 chip.