666

The number 666 is considered in the Christian bible to be the number of the beast and there are various myths surrounding why this is supposed to be unlucky (of note: in other cultures 888 or various other numbers are considered to be evil).

The number 666, similarly to the number 7, is almost certainly related to dice games, which were played regularly prior to the invention of playing cards.

Whilst 1-die and 2-dice games were popular, also 3-dice games had a moderate degree of popularity at the time. Whilst 1-die games would rely you trying to beat the roll of your opponent (e.g. you roll a 6 trying to beat their 4), and 2-dice games are typically about predicting the total, 3-dice games are, again, typically about trying to beat your opponent. A typical 3-dice game would be one player rolling 3 dice and hoping that the majority of their dice (i.e. 2 of them) beat the opponents. For example, if you roll 4,5,3 and your opponent rolls 6,3,2 then the 4,5,3 wins. Ties would lead to rolling again. As is fairly obvious, in such games if anyone ever rolled 6,6,6 then they would be guaranteed to win.

The chances of rolling 6,6,6 with 3 dice in a 3 dice game are 6x6x6 = 1 in 216 chance. As such, if it ever happened, the suspicion would be that the player who rolled it had cheated somehow, perhaps with loaded dice.

The suspicion amongst gamblers playing or watching the game would be that the person who rolled 666 was probably cheating. It may have been difficult to have proven the cheating, and as such they probably simplified things by saying that they were possessed by the devil, and as such punish them regardless.

As with most numbers in mythology, the real answer for why the numbers are considered to be significant lies in games.