If 3 families own a beach house and 2 of those families want to take weeks by themselves, but the 3rd family doesn’t want to do this, is it a two against one decision?
I’m not quite clear on what the issue is. Is family #3 planning to use the house all the time, and doesn’t want to be excluded? Or do they just not want/need to use the house alone, and don’t want to feel like the other families are getting more than they do? In either case, the way these sorts of disputes get resolved depends on a number of factors, starting with the way the house is actually owned.
When you say the property is owned by three families, that could mean that three people (one from each family) are listed on title, or that each family has multiple people on title, or each family has an entity (such as a Family Limited Partnership) that is on title, or that there is one entity on title, but the entity is owned by members of the three families.
The real answer, of course, is that the disputes get worked out between the individuals who have authority to act on behalf of each of the families. But if there is a standoff, what happens to the minority interest? Well, the method varies between the different types of ownership, but there’s usually a way to get out. For example, if individuals are on title to the property, any of them can demand partition of the property in order to get out of the co-ownership if they can’t come to an agreement.
So, if they can’t talk it out, family #3 would do well to consult with an attorney, just to find out what their other options are. Bring the paperwork regarding the ownership of the house, and the attorney should be able to give you your specific options.