What is this equation for?
I fear that without additional information it will be difficult to provide you with a definitive answer. The $x$ usually represents the mole fraction and in the context of a 2-component alloy this seems plausible. The $C$s might indeed be specific heat constants, i.e. $C_{3}$ would be the specific heat of the alloy and $C_{1}$ and $C_{2}$ would be the specific heats of the components making up the alloy, whereby component 1 would have a mole fraction of $x$  and component 2 would have a mole fraction of $(1-x)$. For a mixture of ideal gases such a formula would be correct but I doubt that it will work well with alloys in the real world. But if this came up in the context of a school lesson then the approximation might be ok.