First read What is a color in Processing?

So a color is basically four values: ARGB.

You can mix in many different ways. You say you want to add three colors and divide by 3.

Code Example

1. color c1 = color(255,128,128);
2. color c2 = color(128,255,128);
3. color c3 = color(128,128,255);
4. color mix;
5.  
6. void setup() {
7.   size(400, 400);
8.   noStroke();
9.   fill(c1);
10.   rect(0,0, 100, height);
11.   fill(c2);
12.   rect(100,0, 100, height);
13.   fill(c3);
14.   rect(200,0, 100, height);
15.   mix = mix3(c1, c2, c3);
16.   println("mix rgb: " + red(mix) + ", " + green(mix) + "," + blue(mix));
17.   fill(mix);
18.   rect(300,0, 100, height);
19. }
20.  
21. color mix3(color c1, color c2, color c3) {
22.   float r = ( red(c1) + red(c2) + red(c3) ) / 3;
23.   float g = ( green(c1) + green(c2) + green(c3) ) / 3;
24.   float b = ( blue(c1) + blue(c2) + blue(c3) ) / 3;
25.   color mix = color(r, g, b);
26.   return mix;
27. }

So you see what happens when you add three colors and divide by three in their respective RGB color channels. In this case you end up with a mixed grey of 170.

Apparantly you have some mix method in mind which somehow mixes into a color (64, 98,159). Of course this is fine. My advice would be to state explicitly how this mix method should work and then write it in an algorithm.