Since the function $f(x) = (a-2)x^2 + (a-1)x + 3$ is an even function,
it follows that $a-1=0$.
Therefore, $f(x) = -x^2 + 3$, whose graph is a parabola opening downwards and symmetric about the y-axis.
Hence, the increasing interval of $f(x)$ is $(-\infty, 0]$.
Therefore, the answer is $(-\infty, 0]$ (or $(-\infty, 0)$ can also be filled in).

So, the final answer is $\boxed{(-\infty, 0]}$.