I have done trenches like that 3 ways. The fourth is to put a pto driven trencher on the rear, but that is too expensive for most of us.
First way, I welded a small cultivator plow on to one of the scarfiers on the box blade and opened the trench with that. Hard to keep the trench clean enough.
Second way, I cranked the tilt on the 3 point as far as it would go with a box blade and made multiple passes cutting an angled ditch.
Third way, I used a rear blade, turned it forward about 45 degrees, and tilted it down 30 degrees and dug about 800 ft of trench about 1 ft deep and very clean. Hard to set the amount of bite with the blade, and too much will bog down the tractor.
Bird, I believe has done this with a middle buster plow, but I don't have one to try.