I agree with everything that has been said here except one thing and that is, do you want to go through the expense of a refractometer for three bottles of wine?
Personally, I would watch the grapes for signs of over ripening, e.g. dehydration, leaves turning, taste and make the best picking decision you can. It will probably be mid to late October from what you have said. I would then harvest them, crush them and measure the SG of the juice and adjust it as necessary with simple syrup to the level of potential alcohol you desire. You can even back sweeten it if you desire.
Again, everything said here is correct and if you were talking about 20 or 30 gallons of wine, investing in a refractometer would make sense. For this amount of grapes, take you best shot. If it works, great. If not, make wine vinegar then take the $70 or so dollars that you would have spent on a refractometer and buy yourself 4 or 5 bottles of wine.