Adam Clarke on 1 Samuel 31:9

And they cut off his head - It is possible that they cut off the heads of his three sons likewise; for although only his head is said to be cut off, and his body only to be fastened to the walls of Beth-shan, yet we find that the men of Jabesh-gilead found both his body and the bodies of his three sons, fastened to the walls, 1Sam 31:12.

Perhaps they only took off Saul's head, which they sent about to their temples as a trophy of their victory, when they sent the news of the defeat of the Israelites through all their coasts, and at last placed it in the temple of Dagon, 1Chr 10:10.