I didn't really bother looking at other robotic hands, because I am one of those people who would rather re-invent the wheel and learn a crap ton of miscellaneous things than just copy someone else and not fully understand every aspect of the product.
The human hand I am talking about is exactly the same size as my own hand, and all 20 actuators fit inside it, assuming you use steel or aluminum bars for the "bones", and you place the actuators very carefully. My hands are not overly large, I am ~ 6' 1" and I wear like a 7.5 or 8 ring size on my right hand. Left is a tiny bit fatter...and being as I'm left-handed, I designed that one first. I actually carved a 3 digit finger out of .75" diameter dowel rod to simulate my contraption. The wooden finger is exactly twice the size of the one I will be building for my hand. It isn't put together yet, as I just moved across the country, my drill is buried in about 5 million boxes in the garage, and I broke all the blades for my Xacto knife...LOL. Once I get everything settled in, I'll finish building it, and I'll probably build the whole hand out of wood and twine just to demonstrate the concept. I'll upload it to my website (once I get that up and running), and I'll put a demonstration on there for people.
Now for the three pronged gripper.
Imagine, if you will, that there is a box, whatever dimension you want, with a rod coming out of it. this is the body of the robot and the arm of the gripper. If you make the arm the same length as the length of the body, you can fold it down on top of the body and it will sit flat (as long as its inside a grove on the top of the body).
for more flexibility, you can put a pin at the end of the rod, and attach another rod to it. This will be the entire arm. At the end of the second rod there will sit a metal disk. This metal disk can be any diameter you like, as long as it is at least the diameter of the rod it attaches to. Here is where my explanation gets tricky.
If you look at your disk from the top(you see the round area, not the edge), you can place three small rods 120 degrees apart along the edge of the disk. these rods will act like fingers, and will rotate on a simple metal pin. You can make them open as far as you want by notching the disk in different ways, but thats a different story. For now, imagine that the farthest these can open is to be parallel with the top of the disk. you can add one more rod to the end of each small rod, just as you did to the arm of the gripper. This will provide more dexterity and will allow you to grab a wider variety of objects.
**Important note** All fingers must be solid rods, whereas all arm components must be hollow.
If you take the rod for the finger and drill down the central axis, in an off center position, you will effectively create a finger once you run a cable through it. You do the same thing with the second piece of the finger, taking care that the holes line up exactly, and you have made the "bones" of a finger.
run a steel wire through the hole, making sure that its a snug fit (perhaps put a straw in first to lower friction), and secure the cable to the tip of the finger with solder, welding, or your preferred method. The important thing is that the cable cannot fall out of the hole. because you have drilled your hole off center, when the cable is pulled, the finger will bend, just like your own. To straighten it out, simply push on the cable. if the hole and cable diameters are close enough, you will be able to push on the cable as if it were rigid.
The reason the arm pieces have to be hollow is because that is where you will mount the actuators and motor. You can make a mounting, where each actuator will be placed inside, and attach it to the underside of the disk. you can then attach your servo to the mounting, and it will allow you to rotate the wrist without ripping the wires out of the actuators. Granted, you will have limited motion on rotation because of the wires, but it will suit you for most purposes. as for the arm, you will need a linear actuator mounted inside the lower portion in order to bend the upper portion. you may choose to move it in a similar fashion, or you may choose to have multiple actuators allowing for more precise movement. you can have a servo inside the body of your robot controlling the pitch of your arm, and another controlling the rotational movement.
This is how I would make a three pronged grabber. All of this can be easily controlled with a PLC, however, I do not know if real-time control is possible using that method. a custom I/O interface may be required, but that is a different matter, of which I am not qualified to advise.