My reliable and steadfast Apple Watch 3 has only rarely suffered from battery drain glitches. When these have occurred, I have generally found that - though potentially triggered by a new Watch OS version - the underlying culprit was some app that wasn’t happy with that upgrade. My watch usually has about 60% charge left when I slap it onto the charger at night. So when it recently started running out of juice completely by midday - I found my way here and learned that all the “usual suspects” had already been investigated and weren’t identified as useful fixes. At that point, I decided to try eliminating apps that might be troublesome. I found removing one app in particular completely fixed everything and restored my watch to its usual happy rate of battery consumption. That app: Outlook. (I had it set as the center complication on my watch face, too.) I don’t know if this might be an issue for others, but if your battery suddenly started crapping out by midday, and if you have Outlook installed on your watch, it might help to try removing it entirely. I hope this is useful.