Why is the signal rate separated into three case?
Even if the transmitter sends the same "signal" (the more common term is "symbol") several times in a row, it does not mean that the signal rate (baud rate) has changed.

However, if the receiver is relying on signal changes in order to perform clock recovery, long strings of the same signal could present problems. Many communications systems employ data scrambling circuits in order to minimize the probability of this happening.