Which are the ways to access "Now Playing" in WatchOS 3?
As of watchOS 4, you have a few options for accessing the Now Playing screen:


Set the Dock Ordering setting for the Dock (Watch App on iPhone > Dock) to Favorites and choose the Now Playing "app" as one of your Favorites
Add the Now Playing complication to a watch face (unfortunately, it's only available for Complication sections that are big enough to show the title of what's playing)


I actually included this in a roundup of things I think should work with Siri on the Apple Watch but don't.