So, I've played 2 games thus far with all 3 boards in play. I also play solo with 3 investigators and some optional rules like Too Many Secrets and the Choose 3 Items when shopping. I have been sufficiently thrashed both times I played against the Vampire with all three boards. In fact, in the last game, the Vampire had 6 evil elders with him and a combat score of 10 or 11, wounds over 25, and I only got 12 turns before I was essentially forced to form a Showdown and Hunting Party. I think adding 3 more elders makes this game really hard, especially with the Too Many Secrets rule.
The added boards/Elders haven't increased the difficulty too much more for me. But I have found the new Villains that I've fought thus far as big a jump in difficulty, or maybe even a tad more, as SW.
The total of 3 boards CAN have a tendency to spread things out more & make you work harder to gain Investigation & fight Minions. But then again I have seen things fall where everything stayed fairly localized to one board too.
Mostly, it's forced me to alter my tactics some & learn some different ones. ---MS