1)

-1/8a²+1/4ab-1/8b²  =  -1/2(1/4a²-1/2ab+1/4b²)=  -1/2(1/2a-1/2b)²

2)

3a(b-4)-2b+8    =   3a(b-4)-2(b-4)   =   (b-4)(3a-2)

3)

X³+3x²-x-3   =   x²(x+3)-(x+3)  =  (x+3)(x²-1)   =   (x+3)(x-1)(x+1)