解:$(1)$由题意,将点$(2,$$-4)$的坐标代入$y = kx$中
得$2k = -4,$解得$k = -2$
∴$k$的值为$-2$
$ (2)$由$(1),$得$k = -2,$∴$y = -2x$
将点$(-1,$$m)$的坐标代入$y = -2x$中,得$m = 2$
∴$m $的值为$2$
$ (3)$由$(2),$得$y = -2x,$且$-2<0$
∴$y$的值随$x$的增大而减小
又$-2<\frac 12<1,$∴$y_{3}<y_{1}<y_{2}$