“One stroke of good (or ill) fortune is often followed by many other instances of luck (or misfortune) when you least expect them.

The proverb dates back to the eighteenth century. In 1726, English physician John Arbuthnot (1667-1735), published a book entitled ‘It Cannot Rain But It Pours.’

Jonathan Swift (1667-1745) and Alexander Pope (1688-1744) collaborated on an essay entitled ‘It Cannot Rain But It Pours.’ The saying has been use ever since.

E.g.

What with all the recent disasters happening in the world right now; typhoon in Taiwan, hurricane in Americas and earthquake in Peru, this is a good time to declare, “it never rains but pours”.

or in another way to say it..bad things happen in 3’s. just saying things happen in groups.

It means that nothing ever happens in small doses – if it happens, it happens in a big way.