Seclusion in separate units

There are three separate rental units, which are all behind one outside door. This outside door is locked by a combination lock. What’s the halacha when in one of the units lives a woman whose husband is out of town, and in another unit lives a man?

This is indeed considered a forbidden seclusion. What should be done is this. You should make the main door openable from the outside. For example, you could give the code to the door to one of the neighbors, and have him actually open this door once in a while.

See Shulchan Oruch, section Even Ha-Ezer, ch. 22