watchOS 4.3 (build 15T5209a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm), released on 2018-03-12T20:07:56Z. The update package is 331.38 MB. This is a delta (partial) update that requires 4.2.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 615b81bc16bf2941a6248ade1b14b011d14f7620.zip.