How many cases have you opened? I do about two each release. I've been doing this for 8 years. I get one foil mythic. Per case.

Here's MTGGoldfish's by the numbers: a typical box will contain 0.25 foil mythics (one per case), about 1.5 foil rares, and 6 or 7 foil uncommons

.25 x 4 is 1.00 1 per every 4 boxes. I don't know what product you're opening where you're pulling 3 FOIL Mythic Rares per case, but it's not magic. Things like printing errors are what messes with randomization.

Everyone knows that the cards aren't completely randomized. It's why box mapping is a thing that still happens.

So maybe you're not wrong when you say it's 'possible' to get additional FOIL mythics in a case. It's typically due to a printing error.

Wizards disclosed the odds on both the foils and most recently masterpiece and expeditions. Both of which were number crunched to one per case. 1:67 cards produces a premium version.

It's rare to get more than one foil mythic per case. It would be even rarer to not get one foil mythic per case. Also when we're opening boxes and noticing things like upside down cards, can rares that are out of order and out of sequence it kind of points to an error in printing vs variance.