Is there an app that I can use on my Galaxy S8, will download to my Gear S3 and be able to text from my Mac ??
Yup, Pulse! They have an app for everything! (Pretty sure they'll release one for fridges and microwaves if they have their way haha) Or Google's Messages (I don't know if you can download to a Mac and I wanna say yes but I'm not 100% sure if that downloads to a non-Android Wear watch like the S3, but that works over any web browser).