פירוש על עבודה זרה 94:11

The problem the Talmud has is that the Mishnah says that if he built the house for idolatry it is prohibited. By implication, if he built it and then worshipped it, it is not prohibited. In such a case, since it is already attached to the ground, it does not become prohibited.
The resolution is that the house becomes prohibited whether he built it for idolatry and then worshipped it or if he worshipped it but did not build it for idolatry. In either case, the house becomes prohibited.

To Rav there really should be four clauses in the mishnah: one where he built the house for idolatry, and another where he built it for other purposes and only later worshipped it. So why are there only three?
The answer is that when it comes to annulling the house as an idol, it does not matter whether it was built for idolatry or only worshipped after it was already built, the tanna of the Mishnah included both categories in one clause.