If it does not appear in the Play store on the same page with the phone app (usually there's an option to install on other devices) it cannot be installed. The Watch 3 uses the Tizen OS, so likely you cannot install Google Messages on it. However, you should still be able to receive the notifications and texts on the watch. You will just receive and have to respond to them through Samsung Messages. Both Samsung Messages and Google Messages are just readers and your messages appear in both concurrently. You just have to make one or the other your default on any given device, as thy cannot be *used* concurrently on a single device.
However, you should also be aware that Samsung is discontinuing all support for Tizen by the end of this year, so if something goes wrong, you will have to upgrade your watch.
