Josheb-Basshebeth, Eleazar & Shammah are listed in 1 Samuel 23:8-11. Only 2 are in 1 Chronicles, Jashobeam (a shortened form of Josheb-Basshebeth) & Eleazar. The 3rd is not mentioned, but the fact that there are 3 is (v 21). To speculate on why Shammah is not mentioned is pointless. The absence of his name (an omission) is not a contradiction. It seems the questioner, by including 1 Chronicles 11:6 is suggesting that Joab was among them. He was not. Joab was over all, as the commander of the army (2 Samuel 8:16; 10:7; 1 Chronicles 18:15; 19:8).
Adam Clarke’s Commentary cites Kennicott, who indicates the eight hundred is probably a corruption, and that the three hundred of 1 Chronicles 11:11 is correct. As we have acknowledged at other times, the original text was given by inspiration, but it is possible for copyist errors to enter through the process of time. The Hebrew word for three (שלוש) was corrupted at some point and rendered as eight (שמנח). A discrepancy exists between the accounts on the number of men slain at once by David’s mighty man. The skeptic will make a big deal about it, whereas if the same were found in non-biblical histories, not another thought would be given to it.
Source: #173 & #174, 1001biblecontradictions.com