Quite an easy fix:
+ System: Every 3 seconds
-> Sprite: Set animation frame to Self.AnimationFrame+choose(-3,-2,-1,1,2,3)
You can change the range to be as big as you need, don't use random as that will include 0, though in fairness, you could use a sub event in that case.
You'll also want to clamp the above, otherwise you could end up with minus number frames