Do you intend to use the 3ph arms to lift the weight with the boom pole? Or hang a lifting device (eg: chain falls, come-alongs) from the boom to do the actual lifting?
The maximum rated capacity of the 3ph at the outer ends of the lift arms is 400 lbs. Of course, since attachments generally extend past that point, the maximum attachment weight is less -- for example 300 lbs for a rotary mower or 350 lbs for a snowblower.
The leverage that a boom pole can apply will depend on how far back from the pins that the pole extends, and includes the weight of the pole itself. If your pole is 100lbs and rises straight up from the pins, you could lift 300 lbs. But if it extends 5 feet rearward, you could lift 60 lbs.
The issue is not just the capacity of the hydraulic cylinder, but also the strength of the tractor, notably the top link attachment point. Kubota offered a heavily-reinforced top link bracket for use with a back hoe on the B7100 for this very reason. Without the optional bracket, the casting could be broken off when subjected to the leverage applied when operating a backhoe.
I would suggest that you design or modify your boom so that it rests itself on the ground in operation, using the 3ph for transporting the boom itself only, and do not attempt to move the tractor with any significant weight hanging from the boom.