I have in play Inflation, and 1,2,5!, so that's really 2,3,6!
I play Draw 3. Applying my 2,3,6, nothing happens. Inflating it,
makes it Draw 4.
I still only applied each card to it once.
It doesn't matter that you can find a case where one doesn't do anything. In a theoretical case where Hand Limit 1 and Hand Limit 3 are both in play and I apply the HL1 by discarding down to 1, the HL3 then doesn't do anything. That doesn't mean they both get to stay in play.
Here's Andy's explanation from the FAQ:
Q: How does the New Rule One, Two, Five! interact with the Inflation rule?
- A: They contradict each other. Since the Inflation rule changes all numerals (by adding 1) and One, Two, Five! also changes numerals (by changing 3s into 5s), these two rules just don't play nicely with each other. Therefore, if you add One, Two, Five! to a deck containing Inflation, and they both get played, the first of the rules played must be discarded when the second rule goes onto the table. (It's like when a Draw 5 rule is replaced by Draw 3.)