...usually the horse has left the barn means it's too late to hope for cure. T3 certainly doesn't mean it's too late for cure. But at every new piece of information when the news could have been good or bad it was always bad.
I'm pretty sure that "the horse has left the barn" simply means that the cancer has escaped the prostate, i.e., extraprostatic extension. No implications regarding terminal status.
Argent, I'm sorry about
your horse. I did many of the same chores you've mentioned, but as a teenager. I think growing up on a farm where we often had to fix everything ourselves is why I decided to become an engineer.