Well, scarecrows are used as decoys right? So perhaps your scarecrow character's attacks should be based on decoy and deception, such as teleportation behind your enemy. He might even attach his enemies to a post to render them defenseless, or spit straw at his enemy like a machine gun. I think this character would have a strong backstory as an ordinary scarecrow given life through dark arts. He could even be a dark twist on the Wizard of Oz.
Snowmen might obviously have ice-related attacks. One such attack might be freezing your opponent's head in a block of ice and punching them, thus shattering the ice. This might also work as a finisher, busting their head like shards of glass. He might also be a tactical fighter with strong deadly ice attacks using his own body. The attacks are powerful but every attack lowers his hp. I just wonder how you would go about animating the snowman? His back story may be similar to the scarecrow's, or he could be a "good guy" or "protector" if you will.
A Prisoner fighter might specialize in chain attacks, that can pin down your enemy and render them defenseless, or bring them closer to you (similar to Scorpion's "get over here" move). He might also have a small shank that he can stab an opponent with, doing small amounts of damage over time.
Now the clown could get really dark really fast. He could have a finishing move where he ties his opponent's intestines into the shape of a poodle, or squirt acid out of a flower and smash their head in with a mallet. He could also pull a string of handkerchiefs out of his pocket and strangle his opponent with them. He might have a fighting style similar to a loose cannon, being unpredictable and able to absorb a lot of blows.