Ok here goes then,
Slann
At the moment he looks one dimensional, your shading looks ok but your highlighting can't be seen, try doing extreme highlights on the top of objects to catch the eye, the veins need shading and then a layer and highlight again take your time and look at each stage objectively.
The Slann needs spot highlight to help bring out shaping, knuckles, knees, elbows etc again go extreme.
Warriors
The wash on the white is too dark creating a dirty look this may be intentional, the green looks flat and needs a shade and highlight, shield the shade is the wrong tone for the blue it's too strong atm, if you would have done a dark blue base then highlighted up to your current blue that would have been perfect, claws look too white again shade and highlight don't be scared to experiment.
Carny
Now your at an early stage here so hard to see where your going with it but the wash is too strong at the moment when your going over white you really should thin down your wash and apply thin coats to build up the colour.
I've said this before but never be afraid to make mistakes and experiment that's how we all learn.
Now I sound like an expert and banging on but I don't mean it like that and I'm far from an expert, I want you to get better and love helping so if you feel I'm coming across too strong I apologize.
Let me know if you want to go through a step by step say on the green of your warriors.