You have four points and a triangle has only three vertices. Did you mean "quadrilateral"?

You have completely ignored (0, 0, 0). Please state the problem as itPQ (-1,0,1)

Coord xyz of Q minus coord xyz of P

PR (0,-2,3)

Coord xyz of R minus coord xyz of P

Then apply 1/2 |square root a^2 + b^2 + c^2| = square root of 17 / 2reallyis!