I've gone through similar growing pains... from Simplicity garden tractors to a Kubota SCUT and then a Power-Trac PT-425. I kept the Simplicities all the while (though I did own one with a factory FEL for a while, then sold it) because I have a 1/2 lot and the larger machines were for prepping my 5 acre retirement site. It took me two tries there (actually three, because I originally thought the Simplicity with FEL wight be of value) to get the right equipment package for my needs.
I've had a 32" tiller for the garden tractors and now have a 46" (actually a 36" plus optional tine extensions). I find it is of limited value on my 1/2 acre lot, and is only used twice a year at most, for prepping my garden in the spring and tilling things under in the fall, as stated in a different thread. I use my 16" Troybilt Pony far more in the garden because I can cultivate with it, once planted.
My advice would be forget the tiller for now, and certainly don't invest in the 3pt just to add one. It'll be too big for most of your needs, anyway.
Instead, rent a large tiller or pay for someone local to expand your garden and focus instead on how you'll cultivate and maintain that garden, once you've expanded it. That points toward a walk-behind, and IMO a rear-tine one, not the front-tine one that'll beat you to death (yeah, I tried one of those also, before buying the Troybilt 20 years ago).
You can always keep an eye out for a bargain on a used tiller designed for your JD, but like I said, you'll likely find it of limited use...
IMO, save your money and save your JD, by looking at a smaller tiller for your ongoing needs...