The more I have thought about this the more I believe that actually no, you should not be able to loop this. However my opinion is invalidated by the fact that Scott Marshall has replied to my inquiry with this ruling made by Gavin Duggan (L3). I would thus feel comfortable using this in an event under this understanding.



"tl;dr - Yes. State that you're going to cast it 410,758 times, and then you're you're guaranteed to have the three cards you want in a row. No math needed at the event; 410,758 will always get you what you want.

Long version: Fortunately, the number isn't theoretical, it's finite and fixed. You need to specify the entirety of the final result to avoid the “Four Horsemen rule”, but you can do that by saying that the outcome is “these three cards in a row, followed by the rest of the deck sorted by collector number.”

Unlike the Four Horsemen combo, which requires a random process (shuffling), that arrangement can always be achieved in at most 410,758 castings of Petals of Insight (assuming there are less than 60 cards in the library, and the number of cards is not divisible by 3). So if you cast Petals 410,758 times, your deck will be sorted in the
desired order. It probably needs far less (duplicate cards, chance pre-ordering, less than 59 cards in the deck, etc) but if the deck ends up sorted in less than the maximum number of castings, you just go through the remaining iterations without changing the order of the deck… so 410,758 will always do.

So,

* the number of iterations is finite and fixed
* the outcome is deterministic
* executing these loop iterations takes zero play time, same as any other loop (See the linked August 2013 ruling)

That that's good enough for the loop rule to apply. Once you've done so, you just need to manually cycle through the deck to put the three cards you want on top. That can require up to one more casting of Petals for each card in the deck, each time putting the top three cards directly onto the bottom in the same order if they're not the
three you want. This part can't be covered by the loop rules, but takes less than a minute and the game state is advancing towards a fixed end, so it's not slow play."