FYI for whoever this might help, I just took apart my cellular series 3 watch, this one is from the first year of the series 3 (when it was the newest watch). There were 2 differences, one of them didn't matter, but one of them was of critical importance.
I bought the iFixit kit for this battery replacement. One, rather than the panel being a separate black panel like in the pictures here, it was one continuous black sticker I had to peal up.

Secondly, the side panel that they say to use a Y000 screwdriver on, I tried using the Y000 tip included in my iFixit kit, AND IT DID NOT FIT. luckily I had a few tri point drivers around so I was able to undo it, but they are not labeled, so that doesn't help anyone else in this situation.