JJ,
What do you mean by you already have a subframe?  Are you talking about the mounts on the tractor it self where the attachment pins are installed? -OR- Are you talking about the two long steel arms that go from the backhoe itself under the tractor and attach just forward of the bell housing?
I originally bought a used TC-35D with a 16LA front end loader and a 757-C backhoe.  Years later I found a deal on a TC-45D, tractor only.  So I bought the TC-45 and swapped everything over from the TC-35.  I turned around and sold the TC-35 for $200.00 less than what I paid for the TC-45.  Nice upgrade for $200.00.  So I kind of have an idea of what's involved to install a backhoe on a class III Boomer.  If I can help you just let me know.  I'll be glad to shoot some pics for you tomorrow, weather permitting.
BTW....the only place the 3 pt. hitch comes in play here is at the top link mount on the tractor.  You use the top link as a stabilizer/brace back to the backhoe frame.  The lift arms have nothing to do with the installation.