I bought the Series 3 in June last year, knowing there would be a Series 4 in the fall. But, I was replacing a Series 0, I needed the cellular option, and I knew I wasn't interested in the bigger form factor rumored for Series 4. I was happy with my decision then and remain happy with it now.
So, yes, in my view a discounted Series 3 is worth it. And if you don't have an Apple Watch, it's worth it many, many times over.