The Series 3 will reportedly be modernized and rebranded to an SE model, and the 5th will likely be phased out instead of the Series 6 models.
If you are interested in a series 5, there will be a flood of those put up for sale / unloaded after the new clock message, because the upgrade of series 4-5 was more or less a joke.
I love my 5, but I still feel ripped off by it. The 6 will be worth it if even has only one of the new functions as oxygen in the blood
Edit: word choice