Paco . . . Actually you might want to post in the ATTACHMENTS forum not in any of the specific tractor forums for a question like this.
However, to answer your question, you should be able to use a standard PHD with your sub-compact BX but you will find some inconveniences with it. First, I won't lift quite high enough so you MAY find that will have to get out of the seat (with the tractor turned OFF) and manually position the auger bit for each hole.
There has been some discusson on TBN specifically about Post Hole Diggers on Sub Compacts. Basically it seems like they do work, you just have to futz around with them a bit.
To eliminate some of the messing around, the other option would be to buy a standard PHD and standard auger bit, then CUT a few inches off the auger bit, re-drill new holes for the shear pins and use the now slightly shortened bit. Most bits are about 42 inches long, you need to drill 36 inch holes. You probably could take 3" off the bit lenth and easily use it with a sub-compact???
Maybe HENRO can step up with a better answer???