A triple finesse (or "deep" finesse) is a finesse that's taken against three opposing honor cards:
In this layout, South is missing three spade honors: the king, jack and ten. If certain that the king is offside or if needing 3 tricks, South can lead a spade to dummy's 9,
hoping that West has the jack and ten.
Again a spade is led to dummy's 9.
The hope is that West holds one of the following:
In any of these cases, South is assured of two spade tricks.