So the this pact describes that "Your patron gives you a grimoire called a Book of Shadows. When you gain this feature, choose three Cantrips from any class’s spell list (the three needn’t be from the same list). While the book is on your person, you can cast those Cantrips at will. They don’t count against your number of Cantrips known. If they don’t appear on the warlock spell list, they are nonetheless warlock Spells for you." So does that mean i can choose the Mage Hand Legerdemain from the arcane trickster spell list since this counts as a cantrip from a spell list?

That is not a specific spell, but a subclass feature.  It gives arcane tricksters different ways to use their mage hand.