MB4 247a: Multiple Adjoining Yards (378:5) Courtyards Separated By Multiple Houses (379:1-2); Nullifying Your Reshus (380:1)

If you don't have the ability to carry in your own courtyard you prohibit carrying in areas where you have דריסת הרגל rights (e.g. an outer courtyard).

If 2 courtyards are separated by 3 houses and the courtyards are only open to each other via the houses in between. How do they make an eruv? If one house doesn't contribute to the eruv when does it ruin the eruv?

If a person forgets to join in an eruv he ruins the eruv unless he expresses a bitul reshut (which can be done even on Shabbos).