Embodiments of the present disclosure relate to a memory system, a memory controller and an operating method, which allocate one or more of a plurality of buffer slots in a buffer pool to a write buffer as write buffer slots or to a read buffer as read buffer slots, configures initial values of count information on the respective write buffer slots and the respective read buffer slots, which indicate remaining allocation periods respectively, and updates the count information on each of at least some of the write buffer slots when data is written to the write buffer or updates the count information on each of at least some of the read buffer slots when data is read out from the read buffer, thereby providing optimal data read and write performance and minimizing overhead caused in the process of dynamically changing the buffer size.