Complex sentence: These stations were previously co-owned with AM stations which originally used three-letter calls but have since adopted other callsigns; in this case the TV station is permitted, but not required, to drop the suffix.
Split into simpler sentences: These stations were previously co-owned with AM stations which originally used three-letter call signs but have since adopted other call signs. In this case, the TV station is permitted to, but not required to, drop the suffix.