Final answer:
The Korean Three Kingdoms period featured the kingdoms of Goguryeo, Baekje, and Silla. Of these, Baekje was the kingdom that bordered both the Yellow Sea and the East Sea.
Step-by-step explanation:
The Three Kingdoms in Korea which bordered both the Yellow Sea and the East Sea during their respective times were Goguryeo, Baekje, and Silla. Among these, the kingdom that bordered both seas and is mentioned in various historical accounts to have had access to the coastal regions encompassing both the Yellow Sea and the East Sea is Baekje. Baekje was strategically located in the southwest region of the Korean Peninsula, allowing it control over these coasts, which was significant for trade and military reasons. Goguryeo was located more towards the northeast and did have a coastline along the East Sea but did not border the Yellow Sea. Silla, on the other hand, was situated more towards the southeast and mainly had access to the East Sea.