Yeah, they stack, easiest way to test it is on low level mobs when you have three AoE stun, you will be able to perma lock them whatever the combination :  
Flux : 4sec > Shift : 6 sec > Skew : 8 sec > Flux : 4 sec > Shift : 6 sec 
 
if Flux or Shift wouldn't stack with Skew, mobs would attack again when skew wears off which is not the case 
		
		
		
		
		
			
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