For ATPS you can do two things:

-A normal plot in which the x-axis has the salt % and the y-axis, the PEG % such as this http://patentimages.storage.googleapis.com/US7803405B2/US07803405-20100928-D00002.png

-A ternary plot. For this plot you need to use special software like Matlab or Origin that do it for you.

1)There will be 3 components, and 1 variable(Temperature) So 3 free variables, In binary phase diagram we have 2 free variables(Pressure kept constant) so binary phase diagram can be plotted in 2 Dimensions, But for ternary will also need z axis(Usually It is used for Temperature) and in X-Y plane an equilateral triangle is drawn wit each vertices corresponds to pure component, So when we draw ternary phase diagram on the paper  Temperature is also fixed.

2)To calculate composition at any point inside or on the triangle please refer to the following image

(http://chemwiki.ucdavis.edu/@api/deki/files/41507/TRIANG-1.png?revision=1)

First u need to plot it in 2D like triangle and add the purity of the three element at each corner and draw the binary plot between each two element after that now u should go for three dimention plot depending on the binary diagram u draw first