How do you do Pythagoras theorem in 3d?

Answer 1

d2 = a2 + b2 + c2

a, b, and c are dimensional distances which are perpendicular (like x, y, and z coordinate system) and d is the distance between 2 points separated by those dimensions (or a diagonal line between the 2 points).

Answer 2

A^(2) + B^(2) + C^(2) = d^(2)

Where A , B & C are the linear dimensions of the object. 'd' is the diagonal from say, top right back corner to the bottom left front corner.