If one family wants to sell their share, do the other two families get first refusal, or right to veto another party?
How is the valuation done if one family does need to sell its share?
Who gets what weeks - Christmas, school hols etc? What happens if those weeks aren't used? can they be rolled over to the next year?
If it's overseas, it could be the case that you'd need to consider whether disputes would be settled under English law or the law of that country (I don't know whether you can even choose your own jurisdiction, but I'd check).
Who is liable if someone f**ks up and it turns out that the place was built illegally, doesn't have proper title deeds etc? There are an awful lot of sob stories around about people who bought places in Europe and have had them confiscated or bulldozed.
Personally I wouldn't - nothing sours a friendship like money. If you do, each party must get its own lawyer involved from the outset. It's definitely not something that can be arranged over a dinner party.