1)
What is the meaning of "va'Yadbeku"?
1.
Radak: It is like "va'Yadbiku" (Shofetim 18:22, they caught). Also "va'Yadrechu" (Yirmeyah 9:2) is like va'Yadrichu. Malbim (Shmuel I, 31:2) - it does not say va'Yasigu, for Sha'ul fought with great strength.
2)
Why did these three sons die, but not Ish Boshes?
1.
Radak (Shmuel I, 31:2): These three went to the war. Ish Boshes did not.
