Two thoughts come to mind: what soil type do you have, and how much money are you willing/able to spend?
My
BX2200 with Woods BH6000 can dig fine in the clay when it has light moisture content. I can dig in the hardpan only when it has been soaking under water for a couple of months in the winter, and even then, can dig one inch at a time. With the Bro-Tek ripper, modified to have a sharp point miniature tiger tooth on the tip, I can cut a 2" deep groove. In both cases, the bh really throws the tractor around a lot.
If I had some more money to spend, I would buy the biggest B series tractor that will cut while the MMM is in the fully lifted position, and put a bh on it. If money were no object, I would buy a ZTR, or even an F series mower for the grass, and have a full sized utility tractor for FEL, BH, brush hogging, and other heavy work.
If you have sandy or silty loam, and no huge stumps to dig, blackmajicwoman's thread shows what the BH6000 could do on even a
BX1500. If you have caliche, ironpan, duripan, or any true hardpan (fragipan, clay packed by heavy equipment, is not true hardpan), you will have difficulty with a BH6000 on any sized BX.
Having two machines to do widely different tasks can be a wise choice. A person certainly has the right to spend his money any way he decides is right for himself and his family. Nevertheless, the only way I could only see having three machines in the same class as a practical thing, for a majority people, is if he owns a business and has three employees producing income by operating all three machines at the same time.
I hope this helps, but ultimately, only you can decide.