Hello,
Yes, I have developed with the Sony Smart watch 3 using B4A. I did not encounter any problems. I'm fairly certain that I used the B4A bridge on the SW3.
This was a little over a year ago.
Take a look at this thread:
https://www.b4x.com/android/forum/threads/wearable-datalayer.49968/page-4#post-339879
And barx has provided some libraries and examples – search the form.
I think I saw that with the later versions of Android there is a problem that will cause these examples not to work, but I also believe that the problem may have been solved. I did not follow up on this.
Barry.