Well I generally make a point of never casting 3 back to back, but depending on trinket procs it may have happened.
Looking at my
log from malkorok, I cast 18 chaos bolts but only saw 53 ticks of the dot, so was 1 tick short.
I'll have a look at some other fights and go hit the dummy for a bit.
Edit: My reasoning is that the dot normally ticks 3 times, if it is clipped it jumps up from 1 to 4 seconds. However if you cast a third chaos bolt which lands when the dot is at 2 seconds, it only jumps up to 4 seconds again and not 5.