I'd want to know if it's possible to play with Dunwich, Kingsport and Innsmouth Horror expansions at the same time. Thank you!

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Though each expansion is principally designed with just the base game as a consideration, you can certainly use any combination of them. But using multiple board expansions also changes the rules slightly:

For each expansion board used beyond the first, you count the game as having 1 fewer investigator (except for final battle). When using all three board expansions, in a 6-player game, you would set your town/monster/gate limit as though there were 4 investigators playing. This includes rumors that count investigator numbers to operate, and the number of gate trophies required to win when all gates are closed.

Additionally, when both Dunwich and Innsmouth are used together, the maximum gate limit is increased by 1.

Though, the issue of dilution of effects, principally that of activity in Dunwich in order to waken the Horror, starts to become prevalent when you use more and more expansions. Innsmouth's Deep One track gets affected as well. Kingsport, on the other hand, does not get affected at all. Its threat trigger was genius.

Hope this answered your question.