Real world situation with System of Equation with 3 variables? Where do you run into a real world situation involving 3 variables and 3 equations?  Can someone think of a specific example from business, etc?  I recall taking an operations research course that seemed to involve optimization of 3 variables, but do not recall a single example or theme.  Any help is appreciated.  
 A: In the spirit of Christmas and New Years' resolutions, suppose that what we were on a diet and needed to eat precisely $245$ calories, $6$ grams of protein, and $7$ grams of fat for breakfast. Unfortunately, I open my cupboard to see that all I have is three boxes of cereal: Cheerios, Cinnamon Toast Crunch, and Rice Krispies. There nutritional information per serving is as follows:
Cereal                     Calories     Protein     Fat
Cheerios                   120          4           2
Cinnamon Toast Crunch      130          3           5
Rice Krispies              105          1           2

Now, normally, I would dive in and gorge myself on Cinnamon Toast Crunch$^\dagger$ because they're delicious - but, I need to stick to my new years resolution. 
First, I denote $c = $ servings of cheerios, $t = $ servings of Cinnamon Toast Crunch, and $r = $ servings of Rice Krispies. Then, I form the following system of $3$ equations in $3$ unknowns:
$$120c + 130t + 105r = 245 \ \text{calories}$$
$$4c + 3t + r = 6 \ \text{grams of protein} $$
$$2c + 5t + 2r = 7 \ \text{grams of fat} $$
Now, I leave it to you to find out if I am stuck with one bland mix of cereal$^1$, whether I will be able to form many mixtures of cereal$^2$, or If I will be forever cursed with the dreaded stomach-tire$^3$.
$^1$The system has a unique solution.
$^2$The system has infinitely many solutions
$^3$The system has no solution
$^\dagger$French Toast Crunch is even better, and baby it's back.
Note: I got the numbers for this (silly) example from here, courtesy of Dr. Dagan Karp.
A: A few examples that may or may not arise in other answers...


*

*When you have a set of data of the form $(x_i,y_i,z_i)$ and you want to fit a model of the form


$$z=a\,f(x,y)+b\,g(x,y)+c,$$
least squares leaves you with three equations in $a$, $b$ and $c$.


*

*Any kind of a 'network flow'. For example suppose that you have a roundabout with three exits and the flow in and out of each exit is known then using the principle that the flow into a junction should equal the flow out leaves you with three equations in the unknown flow between exists.

*When doing a partial fraction expansion --- which occurs in various applications --- you are often left with simultaneous equations.
A: Linear programming.  look HERE ... in fact the first example (rabbit food) involves three variables.
