Speculation tax on leasehold

But not in the case of owner-occupancy.

Google "2.1.1 Drei-Objekt-Grenze bei Veräußerung selbst genutzten Wohneigentums" at Haufe. So it is indeed also in the case of owner-occupancy, unless you can explain "obvious compelling reasons." "Such compelling reasons can be professionally caused local changes, moving to a residence closer to the workplace, greater space requirements due to family growth, separation of spouses or other plausible reasons, e.g. an unforeseen financial emergency."

But now D is not paid solely from the profits of the sales of A, B, and C, right?

Nah, had to give up enough in the divorce. But overall earned a mid six-figure amount... My goal is, when the kids are old enough and no longer come to me, to sell the two-family house and then build smaller again (from equity).

But not for personal use.

Yes, also. Or rather, it depends.
You’re not allowed to link anything here anymore, so copied from Haufe:

"In the 3-property limit, owner-occupied residential properties are generally not to be included. Only in particularly exceptional individual cases, namely in the case of only short-term, temporary owner occupancy, can the property be included in the 3-property limit. But even then, the circumstances of the individual case must be examined closely, because even in the case of only short-term owner occupancy (less than 5 years), the property is not to be included if the seller can explain the sale with 'obvious urgent reasons.' Such urgent reasons can be job-related local changes, moving to a dwelling closer to the workplace, greater space requirements due to family growth, separation of spouses, or other plausible reasons, e.g., an unforeseen financial emergency."

In other words: If occupancy is less than five years, then plausible reasons must be demonstrated.

Google search for "2.1.1 Drei-Objekt-Grenze bei Veräußerung selbst genutzten Wohneigentums" at Haufe.

Quote Haufe: "In the 3-object limit, owner-occupied residential properties are generally not to be included. Only in particularly exceptional individual cases, namely with only short-term, temporary owner-occupation, can the property be included in the 3-object limit. But even then, the circumstances of the individual case must be examined carefully, because even with only short-term owner-occupation (less than 5 years), the property is not to be included if... " ...= obvious factual constraints

Quotation Haufe: "As a rule, owner-occupied residential properties are not to be included in the 3-object limit. Only in particularly exceptional individual cases, namely in the case of a short-term, temporary owner-occupation, may the property be included in the 3-object limit. But even then, the circumstances of the individual case must be carefully examined, because even in the case of only short-term owner-occupation (less than 5 years), the property is not to be included if..." ...= obvious compelling reasons

The crucial point is the short-term owner-occupation.

Put it the other way around: If use is less than five years, you have to provide plausible reasons.

Who doesn’t have plausible reasons for a move? Bigger house, bigger garden, closer to work, higher recreational value for the children, etc.