The main problem with this design is that for the cargo lock to hold an object in place, the cargo lock needs to have enough structural integrity and mass behind it to support the object it's holding, otherwise it breaks. In other words, the immobilising torpedo would need to be almost as heavy as the target to immobilise it.
A slight change to the design would be to have the torpedo attach to the target, and then fire it's thrusters, throwing off the balance of the target and ruining their maneuverability and aim.
Also there's the 3 beams at 90 degrees to each other problem, which would either limit the torpedos effectiveness to only small ships (which defeats the purpose of a torpedo, which is typically an anti-large weapon) or the torpedo needs to be huge, at which point it may as well be it's own ship.
And I know for a fact that people are already designing ships that will attach to their target for the purpose of boarding.