Would a story with three MCs find a ready market?

Ox,
I don't think it's the issue of three main characters that will cause trouble - plenty of books have three main characters. The success of your story will depend on your execution of it, and the perceived relation between the three people. Is the main conflict of the story clear? Is the relation of each protagonist to that conflict clear, or at least hinted at? I had a manuscript myself, with three main characters, which has been rejected numerous times - but the problem (I see now) was never the characters, just that my perception of the main conflict was off, so as a result I didn't know how to give readers cues as to how each character related to the main conflict, and thus where the story was going.

What Juliette said. What matters is whether the reader can discern the connection between the characters and the characters' connection to the conflict. The novel I'm querying right now has much the same issue as yours--three main characters in three different geographical locations for the whole of the book. They don't even meet 'til the end. For that reason, one of the things I asked my betas to look at was whether they could discern the connection between the three characters' storylines, whether it was obvious that the three lines were connected and effecting each other, and how early they sensed the connection. So far, so good.

So I'm hoping it's not going to be an issue just like you are. I don't think it will.

Quite a number of books have multiple MCs who never even meet one another. The Hours comes immediately to mind. As long as the whole story arc makes sense and there are connections (by themes, by relationship, etc.)

Though it may turn out to be longer than a single MC book, as a *reader*, I think I'd enjoy seeing something like this. It's unusual enough to be fresh - and if you can do what the excellent posts above mention, I'd love to see it on the shelf.

If you can pull off the writing aspect of it there is really no limit to the number of characters you can focus on. John Brunner's The Sheep Look Up has something near a dozen main characters whose plots all interweave to some degree, so I wouldn't have problem following three of them as long as the writing is up to par.