Our old tractor, back home on the farm, had a 3pt mount backhoe. I dug miles of trench with it. Never had an issue. To this day, that tractor still uses the 3pt backhoe when needed.
However...
Knowing what I know now, and seeing the destruction that one of those can do to a tractor (first hand), I would convert it to a 4pt solid mount.
If you dont have the technical knowledge or skill to convert the BH yourself, you may want to consider having a welding/fab shop do it. Here is my thinking: you say you can find these backhoes inexpensively, then buy one and take the savings and have a mount made.
That 3pt BH I grew up with, we bought it cheap used, and had some weird mount on it for a pickup (from what I remember). We didn't have the ability or time to convert it to 3pt. So we took into the local steel fab shop and they converted it for us. I dont remember how much it cost (I was 12 at the time!) but it worked. If we knew about, or if it existed, a sub-frame mount, we would have probably gone with that instead.
Recently, I purchased a used BH that came with a ladder style sub-frame mount. That mount would not work for my tractor. The company (Woods) did not make a mount for my tractor with their BH. So I bought the one that came close and modified it to work. I considered hiring a fab shop to do it, and almost did. That is until the mount came in and I found that it was going to be really simple to convert.
You may want to pencil out what it would take to remove the 3pt, get a frame for your tractor, and marry the two. I do not believe anyone makes a 'kit' for this. You are talking custom work.