So adding two components to a graph that are homomorphic to each other (i.e. two identical “islands”) doesn’t change the value of fair komi. So when playing on three 9x9 boards, you should have the same komi as when playing on one 9x9 board.
However personal preference also plays a role. In the “komi needs correction on 9x9”-thread, the point was made that some value may be “fair” in a theoretical sense (that is when assuming optimal play), but maybe in some strength ranges, players don’t know how to utilise playing first and countering komi. No problem with the pie rule! Just adjust komi according to your personal preference. If you enjoy playing first and feel confident giving white 10 points komi, you can go for it.
//Edit: The downside is, it introduces another hurdle for beginners. You have to make a couple of decisions before you can begin playing, and the rules have no intrinsic feature telling you what is a good choice.
This might not always be true if we take ko into account! Even if one player is always mirroring the two identical islands, the other player can use such exchanges as free ko threats.
Assuming superko, it is actually possible to break mirror go without using the middle of the board! It’s hard for me to visualize. So here, I made a demo: Mirror break