Basically since you have three equations and four unknowns, you will be able to reduce your system to one un-known (or one as a function of the other).
Its like the situation in linear algebra where you reduce a matrix to get the simplest solution but in your case you have non-linear functions of the variables.
An example is say
x + y = 1
2x + y = 2
We let y = 2 - 2x and sub into 1 giving
x + 2 - 2x = 1 which gives solution x = 1
Its exactly the same thing except you have non linear terms.