Say smart to Americans. It's American English. Say clever to British people. It's UK English. Say intelligent if only someone is really very clever, and is good at understanding.

I'm all three, and have noticed little difference between them. On a more serious note.... A clever person can take a car apart and learn exactly how it works. A smart person knows the laws of physics and can build a car. Intelligence is the general term for all the functions of the brain. There are a variety of intelligences, cleverness and smartness being but two of them. Abstract intelligence and concrete intelligence another two. In philosophy we speak of intelligence. Life is endowed with intelligence. It is not endowed with cleverness or smartness. God has divine intelligence. He doesn't have divine cleverness or smartness. That's my two shekels worth.

I am not a native speaker but, as I understand these words it's: Clever can also mean something like a clever trick, so an evil plan. Smart can also, more or less, mean pretty, witty, inventive. So more than just high grades. If a girl says you are 'a smart boy', it could mean more than just sheer 'intelligence' she is referring to. If a native speaker disagrees please correct me.

Intelligent is a scientific term. You use it in a more serious context. Clever is used more freely, you can use it whilst being 'cute'. Smart is probably American and means the same as clever.

My intelligent and clever student, Grandmaster Melaminefree is the closest. Smart or 'having smarts' is largely American although we do use smart for clever. The very fine difference between clever and intelligent is that clever has a suggestion of natural instinct, whereas intelligence suggests a high IQ. They all mean having good brains, but there are different colours in them. A fox is wiley, clever. Military intelligence or academic intelligence. A smart person is someone who is intelligent but it's American and it suggests a youthful person to me. Smart in the US is like brainy in the UK.