Does a triple divorce mistakenly or inadvertently uttered take effect, and what is the ruling on that?

This falls under the category of a vain oath (laghw al-yameen) according to Sheikh al-Islam Ibn Taymiyyah and those who agreed with him. Therefore, it has no effect, and neither divorce nor expiation is necessitated, even if the sworn-upon event does not occur. As for the majority of scholars, they view it as a conditional divorce; if the swearer was discerning (mumayyiz), it does not take effect, but if he was not discerning, it does take effect. The more accurate view (al-rajih) is that it is a vain oath that is forgiven, and nothing takes effect thereby. One must be cautious about toying with divorce oaths.