"An Englishman, an Irishman and a Scotsman" is the opening line of a category of joke cycle popular in Ireland and the United Kingdom.
For example, in England the punchline is usually based around the Irishman being stupid, the Scotsman being mean (i.e. miserly), and the Englishman being posh (or a snob but ultimately not the butt of the joke), whereas in Scotland and Ireland, the Englishman will typically be the butt of the joke.
[2] An Englishman, an Irishman, and a Scotsman were in a pub, talking about their sons.
"My son was born on St George's Day," commented the English man.
Jokes about the hard sciences may begin "A mathematician, a physicist and an engineer..."