Google Venn Charts use a somewhat cryptic parameter for the relative size of the groups and their overlapping areas:

&chd=s:93_a___

Where '9' is the size of one group, '3' the size of the other and 'a' the size of their overlapping regions. Clearly Google was trying to come up with a one-character coding scheme, the goal was to map one character to a range from 0 .. 100.

So, after a couple of minutes well spend, I present the Google Venn Chart coding table:

//php
$code = array(  "A","B","B","C","C","D","E","E","F","F",
        "G","H","H","I","J","J","K","K","L","M",
        "M","N","N","O","P","P","Q","Q","R","S",
        "S","T","T","U","V","V","W","W","X","Y",
        "Y","Z","a","a","b","b","c","d","d","e",
        "f","f","g","g","h","h","i","j","j","k",
        "k","l","m","m","n","n","o","p","p","q",
        "q","r","s","s","t","t","u","v","v","w",
        "w","x","y","y","z","0","0","1","2","2",
        "3","4","4","5","5","6","7","7","8","8",
        "9");

The index of the array corresponds to the percentage of the size. Now we can infer, that our example from above, &chd=s:93_a___ means: 9 → 100, 3 → 90 and a → 42. Note that some values have multiple indexes, I guess Venn charting is not that exact of a science.

This example is dealing with 2 groups and one intersections, but there is not reason, why it should not work with 3-group Venn diagrams.