Yes, I know that doesn't add up to three, but it's that sort of a word.
The unlucky fluke is a parasitic flat worm. There may be as many as 24,000 species of them. They practically all live in snails (which isn't too threatening to me, personally) but at another stage of their life they practically all live in vertebrates, that is, animals with backbones, (which is more personally worrying). Flukes are both male and female simultaneously, use their mouths both for feeding and ejecting waste (I told you they were unlucky) and tend to take it in turns to produce young with the help of a mate or all by themselves. They cause all sorts of horrible diseases, sometimes in humans, though only in places where human waste isn't treated.
Well, that's a small mercy.
The lucky fluke is one where doing something done badly leads to an excellent outcome, like holing out at golf after the ball has hit a tree trunk; or else it can be something that happens against huge odds, like digging up a parsnip and finding your long-lost wedding ring encircling it, or getting a photograph of a weasel riding a green woodpecker.
A fishy fluke can be found forming the tail of a whale*:
southern right whale. Photo by Dr.Haus
They can also be found on the ends of anchors; or fluke can be another name for the sort of fish called a flounder:
This sort of a fluke is jolly tasty, so at least gives us some sort of a revenge revenge on the unlucky flukes, doesn't it.
Word To Use Today: fluke. The key to this word is probably the flounder, which by its shape has probably given rise to the use of the word to mean the parasite, and the anchor and whale words, too. The Old English for flounder was flōc, related to the Old High German flah, smooth.
The fluke of luck word appeared in the 1800s, but no one knows from where.
*All right, I know a whale isn't a fish, really. But they're still fishy!