This question was first raised by Ussishkin in 1985. However, it was Finkelstein who strongly pressed the question into the scholarly world, throughout 1994 and 1995. The main point of fact being that no evidence of Philistine culture was found in LB III contexts, and that no evidence of Egyptian culture was found in Philistine Monochrome contexts, it seemed to Finkelstein unlikely these were contemporary. Since an ostracon from between years 19 and 10 (1176-1166 BC, of Ramesses III, of course) was discovered at Lachish, and one from between year 22 and 24 (1163-60 BC), it is clear that the entire Egyptian administrative system in Canaan did not collapse in the 8th year of Ramesses III.
But was coastal Philistia in the days of Ramesses III in the hands of the Egyptians or the Philistines? Such would lead to absurdities. According to his own reliefs, Ramesses III fought against Tripoli (or Abu Samra; Ullaza), Asharneh (Tunip; a little to the N. of Salhab, on the Orontes), and another fort. Kitchen, who believes Ramesses III did not repel the Philistines, denies the truth of these reliefs, claiming they are merely copies of those done by Ramesses II. If Ramesses III was mighty enough to make war against Tunip, he was mighty enough to make war against Ashkelon. Also, the suggestion Ramesses III’s taxmen cautiously tiptoed around Philistia, while Ramesses III simply let numerous square miles of land under his own nose stay in enemy hands without making any campaigns against this land is utterly absurd.
Also notable is the fact that, while imported Mycenaean IIIC:1b is utterly missing from Megiddo, it is sparsely found at Beth-Shean. This suggests the Beth-Shean Egyptian garrison slightly outlasted the one at Megiddo, and that a blockade was implemented by Ramesses III as a response to the events of his eighth year, which was ended after the Sea Peoples took over the coast, perhaps in the reign of Ramesses VI, in whose time the last clearly dateable Egyptian artifacts appear in Canaan (Megiddo statue, scarabs, ect.). However, Mycenaean IIIC:1a appears at Beth-Shean as well, suggesting Megidddo, lacking this ware, was simply not as important as Beth-Shean or, as Mazar states, Beth-Shean, Tell Keisan, and Acco were “exceptional situations” in a period of heavy decline in trade after the first wave. Even more curious is that Beth-Shean of the 20th Dynasty, built on top of a destruction layer, became ten times more Egyptianized than the Beth-Shean of the 19th Dynasty, and was quite long-lived, having two construction phases, with a governor’s palace being built in the later phase, on top of a stronghold in the earlier phase. This itself strongly argues against Egypt somehow being too weak to take over Philistia; how could it manage to not take over Philistine Ashkelon while keeping fortresses to defend property in the Jezreel Valley? The destruction of the Jezreel Valley sites at the end of LB IIB may be associated with a revolt or the events of the eighth year of Ramesses III or both.
Consequently, it seems that the answer to the question “did Ramesses III repel the Philistines” is an extremely probable “yes”.