In each set, the first plague is accompanied by a notation of the time in the morning. It's also introduced by God's speech, when God says, "Present yourself before Pharaoh," and to do this in the morning.

在每一组中，第一次瘟疫都是发生在早上,可以从上帝的话中看出来，上帝说，今天早晨，“到法老面前去“

耶鲁公开课 - 旧约导论课程节选

So each of the first plagues in the sets of three is introduced this way.

每三组的第一次瘟疫都是这样开始。

耶鲁公开课 - 旧约导论课程节选