Here it says that there were 30 Giborim. Verse 39 says that there were 37!
Rashi (23, 39): Every "Sheloshim" in this Parshah, Yonason translates 'Giborim' (it does not mean 30). Of Bnei Yashan, only Yehonason is named; perhaps there were another two or three who were not named, to complete the count of 37 1 .
Radak (24,39): There are 30 Giborim listed, from Asah'el until Uriyah ha'Chiti. Include the first three, the three water carriers and Benayahu, there are 37. Avishai was [counted] among the water carriers, and Yo'av need not be counted; he was the head general.
Malbim: Pa'arei ha'Arbi (verse 35) is the last of the 30 (refer to 23:23:1:2*). Another seven are listed after him.
It seems that Rashi does not count the first three among the 37, so he suggests that there were another three Bnei Yashan. Perhaps there were only two, if one of the water carriers was not named (i.e. it is not Avishai), or if two of them were not named (like Radak; refer to 23:24:1:2), and ha'Ma'achasi is Elifelet's grandfather, and is not counted. (PF)
Do we know who was the third who brought the water?
Malbim: It was Asah'el. He was not with the first five, who were always with David to guard him 1 , just he was among the 30.
I.e. Asah'el was not one of David's guards even before he was killed. Alternatively, he is not counted with the guards because he was killed before David was established to be king. (PF)
What is "Dodo"?
Radak: That was his name.
What is the meaning of "Beis Lechem"?
Radak: This is like mi'Beis Lechem. Likewise, "Hishamru Lachem Alos ba'Har" (Shemos 19:12) is like me'Alos.
