I am wondering if it would be possible to play with only an expansion board.. say.. the Dunwhich board. Sure, the starting positions might not be correct, but that could be modified so that the players all arrive by train in the first round. The reason I ask is because we are seldom more than 3 players, and playing with an additional board with such a low number of investigators does not seem to work very well.
Playing with expansion board only?
The idea has been brought up before, I'm not sure anyone has implemented it. There are a lot of issues that would have to be sorted out, including a severe shortage of Mythos cards with gates opening for just that board, just to name the one that comes to mind first for me.
Of course. Is there such a thread that discusses this? Otherwise, anyone can post their thoughts on how to make such an adaption of the game possible.
Just off the top of my head, issues you'd need to address include not having a hospital or asylum for people who get wounded, only having two or three other worlds to work with (boring), not having shops to buy stuff at and potentially having encounters directing you to locations on the main board which are no longer in play per this system. You can declare empty table space as LiTaS, Sky and Outskirts, so that's easy, but I'm not sure how to handle the rest.
If your problem is that three investigators is not enough to cover Arkham and one small board, I would say a better solution would be to play two investigators each and count it as a 6 player game. No need for house rules or balance concerns and you have more boots on the ground to cover things. If that's unacceptable for some reason, consider house rules to make it easier to travel between boards instead of removing Arkham all together. Allow the deputy's wagon and other vehicles to drive back and forth between Arkham and the other cities as your entire move (you can leave from any space but always show up at the train station of the other city.) Make a "Greyhound bus" that you can pay 2 or 3 dollars from any space to do the same as above without needing a vehicle card. Stuff like that.
Arkham itself is no small part of this game. The game and all expansions are written and balanced with the assumption that Arkham is there. Instead of trying to lop off a huge part of the game's foundation, I would recommend looking at ways to circumvent the specific rules your group is having trouble with.
Yeah, you're right 
Steve-O said:
If your problem is that three investigators is not enough to cover Arkham and one small board, I would say a better solution would be to play two investigators each and count it as a 6 player game.
Or just have one of you play two Investigators for a four-player game. Or make that fourth Investigator a "community" player, which you all control (like a big Ally). Four Investigators should be enough to cover one extra board, and keeps you under the two-monsters-per-gate-opening rule.
best solution is: skip every 2nd mythos phase.
this way you have much more time to travel around. treat all game mechanics as if there were 6 investigators (for example 2 monsters spawn with each gate), except rumors and the final battle. since this makes it alot easier to close all open gates at once, i would remove this victory condition.
note that this is harder than a 6 player game, but alot easier than 3 players.
More problems include: where do investigators start? Where do they go if they're unconscious? How do you track the terror level? Lost in Time and Space? The Sky and the Outskirts?
You'll only be using four other worlds. This means either eight or six gate trophies, depending on whether or not you're using the Lurker gates. That's no good at all.
What's the gate limit? How many seals should you need to win? To what extent do you apply the "multiple-board, reduced-investigator" handicap?
I have tried something similar to this, I played with the three expansion boards, laying them on top of the Arkham board and off-centered so that all the original other worlds were to the side of the expansion other worlds and the terror track, outskirts, sky, and lost in time and space at the bottom still. I removed all the mythos cards that opened gates in Arkham, keeping the ones that opened on expansion boards and all the special mythos cards (two doom tokens, monster surges, next act, etc.). I played Congregational Hospital as both asylum and hospital and Neil's Curiosity Shop as a place to shop from the various shops in Arkham, simply loosing the ability to buy when the terror track reached the proper level as usual. Starting positions I just tried to make as thematic for each character as possible. To try and offset a third of the board not having gates open, any time a monster surge occurred I would role a die, on an odd role the monster surge occurred, on an even role I would flip a random rift token and open a gate there, excluding the Congregational Hospital and Neil's Curiosity Shop. Satisfying rumors at locations in Arkham I would put the marker at Central HIlls Street, evironments with markers needed would be put at Bishop's Brook Bridge. I can't remember everything else I did to make up for things, but if you have questions I can try and remember what I did. So in the end it all worked out okay, pretty enjoyable.