Try playing with different body shapes and proportions, the poses showcase differing personalities but right now they all have samey silhouettes.
it's not ALWAYS a good metric, but usally if you can make a design all black and have trouble differing it from another you should probably take another shot at that design.

Note how when you black out the designs on your characters here, it becomes a bit hard to tell who is who between the two left characters. The height difference on the third helps to differentiate him, however.
while this is usually more of a western cartooning thing, it's a principle that's applicable to just about any medium i feel.
one animated series that does this well even with 'realistic' proportions is Avatar: the Last Airbender, usually through the use of accessories and varied props.

Note how you can clearly differentiate the characters here based on the shape of their outlines, that's the important bit to take away from this ramble.
[edit x7 combo, cripes I had more to say on this than I thought]