Functional equation problem; chain of functions

I have tried more than an hour but couldn't solve it, can somebody please give me a clue?

$$f:\mathbb R\rightarrow\mathbb R$$

$$f(f(f(X)))+f(f(X))+X=3f(X).$$
Find $f(X)$

I know that $f(X)=X$ is a solution, and I know that the function being (obviously) injective might help, but that's all the useful stuff I could gather.