Komentarz do Diwrej ha-jamim I 11:12

he was among the three mighty men He was included in the three mighty men; he was one of them. And these three were mightier than the rest of the thirty three mentioned nearby. Here he counts only two: Jashobeam and Eleazar, but in Samuel (II 23:11), it mentions the third one as well, as it is written: “and after him [came] Shammah the son of Agei the mountaineer”