Question 84
Function $f(x)$ with domain $\mathbb{R}$ satisfies: when $x \in [0, 1)$, $f(x) = 2^x - x$, and for any real number $x$, there is $f(x) + f(x+1) = 1$. Denote $a = \log_2 3$, and then the value of expression $f(a) + f(2a) + f(3a)$ is ______.