Avodah Zarah, Chapter Four, Mishnah One
In Sanhedrin 7:6 we learned that one who throws stones at a statue of merculis (the Roman god, mercury) is guilty of worshipping the idol, for that his how merculis is typically worshipped. Our mishnah deals with the status of stones found next to the merculis statue.
1) Rabbi Ishmael says: if three stones are lying side by side next to a merculis, they are prohibited; if there are two they are permitted.
2) The sages say: if [the stones] are seen to be connected with it they are prohibited, but if they do not appear to be connected with it they are permitted.
According to Rabbi Ishmael, if three stones were found next to the merculis statue, we can assume that they were used in worshipping the statue, and they are therefore prohibited. If there were only two, then we cannot assume that they were placed there for such a purpose and they are permitted.
According to the Sages the issue is not the number of stones but rather their proximity to the statue. Those found next to the idol are prohibited and those found further away are permitted. Even if three are found further away, we can assume that they were not used in worship.