Here's a suggestion:
Determine the equation of the straight line that goes through points 1 & 2. Standard form: y = mx + b.
Next, substitute x3 into this equation to get the y coordinate of the point on this line for x = x3.
Let's call this point 4, with coordinates (x3,y4).
Calculate the vertical distance between point 3 and point 4:
If y3 - y4 > 0, then point 3 lies above the line (to the "left").
If y3 - y4 < 0, then point 3 lies below the line (to the "right").
Hope that helps.