"继而": conj. it indicates that sth treads on the heels of a occasion or a movement. eg. ① 人们先是一惊，继而大笑。 ②这首歌，先是你唱，继而我们跟唱。 "然后": conj. it indicates that one thing happens after another thing. eg. 先研究一下，然后再决定。 "随后": adv. it has the same meaning with"继而". it is always used with "就". eg. 你先走，我随后就去。 in my experience, i think "然后" is used more often...