When should we start catching the BG THEY HAVE BEEN IN POND 3 years along with LMB

You can start catching them any time. You just want to return your biggest and best ones back to the water to continue producing new BG. If you harvest all the biggest and best it takes your best genetics out of the water. So wait till your second generation BG get big enough to eat and take them or take any of your smaller less desirable original stockers out.

You can fish them, but return your biggest and best back to the water. That is what I do at least. Mine also are three.