The Series 3 was a huge step up in performance. I recently sold my Series 4 and went back to the Series 3 because I didn’t care about having the bigger text or the heart rate monitoring. The Series 3 is 95% as fast as the Series 4 and battery life is about the same. Even after using a Series 4 for 7 months my Series 3 doesn’t feel any different.
In fact, at this point with the Series 5 coming this fall, I’d recommend any first time Apple Watch buyers get a used Series 3 and buy the Series 5 when it comes out.
The real risk is that Apple makes future features 64-bit only. The Series 3 is the last of the 32-bit watches. But as of now, that’s not an issue.