Order of otter box for Apple Watch 3, 38mm
Is there a screen protector that will fit under the otter box
Apple Watch Series 3, watchOS 6
Screen protectors are pretty thin … I expect most would fit under an Otterbox? You might just have to try it and see.
I've used these on my devices before, and they have glass options as well as plastic depending on your needs.
https://www.protectionfilms24.com/c/ALL/apple-watch-series-3-2/