None. There is no strategy available to beat the mirror. No matter what type, in gambling the house always wins and mirror is the basic example of it. Whatever mirror the player chooses, the needed numbers (which is number 5, three times in a row) will most of the time be away from or right next to them. However, my personal strategy is to just stick to the same mirrors when playing. Choosing the first three or the last three ones in the shape of a triangle (2 on top and 1 on bottom or the vice versa). So far it didn't work well because, well, as i said above, the house always wins. However if a player keeps spending money, they'll eventually get something good from mirror with 15 points because let's be real here, it's all about chances and chances of getting 15 points from 3 out of 15 mirrors is not high, but it still is possible.
That last sentence actually segways right into my point: Just keep spending money and you will eventually win, there's no way to think better than a robot designed to make you fail most of the time but still giving you small, sometimes big enough prizes to make you keep playing.