The new Apple Watch Series 3 touts LTE-connectivity. Interestingly, it uses its screen as an antenna. The good news is that this did not negatively affect the watchs repairability. In fact, iFixit rates it at 6/10, same as the Series 2.
Not much has changed on the inside. Obviously, theres the new Apple S3 chipset with a dual core processor and a few new chips which are probably the LTE modem and eSIM. Theres a barometer now too, to more accurately track your exercise.
Also interesting is the battery capacity it packs 279mAh or 1.07Wh, a tiny increase over the Series 2 (which had 1.03Wh). It is a fairly big upgrade over the Series 1 battery though, 246mAh or 0.93Wh. (the Series 1 remains on sale, while the Series 2 has been discontinued)
Apple Watch Series 3: an X-ray look Removing the screen All torn down
You can hit the Source link to view the step-by-step teardown, but the summary is this the screen can be replaced, the battery is trickier, any ribbon cables you damage in the process are going to be hell to replace.