Well, the problem is not really "plotting 3 variables". It is "plotting three dimensions on two dimensional paper".

I would recommend drawing two orthogonal axes representing x and z and drawing the y- axis at an angle "into the paper. Then you are going to have to pretty much "eyeball" the point- 7 spaces on the x-axis, 4downon the z-axis and 1 "into the paper" on the y-axis.