I'm almost sure it doesn't affect critical success or failure chances. All I think it does is give more variability in the potency and duration. Each kismet acts to adjust the final result, so if all 3 are in your favor then you have a better prediction than if you had only done a 3 card one but if all 3 are bad then it will be a worse result than if you had just done a 3 card one. This means that if you want to make a prediction that would be equivalent to using another tool then use a 3 card deal, but if you want to roll the dice a bit more use 4, 5 or 6.