Hello 
You should cast Frostbolt before Flurry because Flurry applies Winter’s Chill to the target. Flurry launches 3 bolts and each bolt applies 2 stacks of Winter’s Chill (With Brain Freeze).
Casting Frostbolt before consumes one stack of Winter’s Chill, gaining it’s benefit but because of travel times the other Flurry bolts will overwrite it and have 2 stacks of Winter’s Chill refreshed so you lose nothing about it.
However there is no need to use it if the situation demands movement.
Hope this answers your question