Rook and knight against rook is drawn in almost all positions: the defender keeps the king away from the edge and uses rook checks from a distance. The knight is too slow to help build a mating net.
Rook and knight versus rook is one of the great drawing endings; the knight is simply too slow and too short-ranged to coordinate with the rook in a mating attack against an active defending king. The defender's main rule is to keep the king off the edge and the rook far away for checks.