高兴 means you are delighted 快乐 means you feel happy 幸福 means peace, health, happiness, love...

高兴 and 快乐 both means happy. If you want to say "I am very happy", you can say 我很高兴 or 我很快乐. Here you can use each of them. 高兴 has another meaning which is different about 快乐. 高兴 can mean like. For example, you can say 我不高兴跟你去看电影, that means I don't like to go to see the film with you. But you can't say 我不快乐跟你去看电影, this sentence is wrong. 快乐 can be used as a noun, which means happyness, where you can not use 高兴 instead. For example, you can say 我来寻找快乐(I come to find happyness. ), but you can not say 我来寻找高兴. 幸福 nearly means blessedness, it include the feeling about happyness, satisfaction, affluence and so on.

When you use 高兴, 幸福 and 快乐 as adjectives which describe PEOPLE. They all mean happy in English, but 高兴 is usually used when you are happy with exact sth For example: I'm very 高兴 that you ask me to have dinner together. or when you want to say that you are in a merry mood. For example: I'm very 高兴 today. 快乐 is usually used when you just FEEL happy. For example: I feel very 快乐. 幸福 is used when you are satisfied and the happiness is usually stronger than 高兴 and 快乐. For example, the person who you love tell you he/she loves you too, and you will feel 幸福.

It is diffcult to explain them. But we can put them into sentenses, then U can feel the difference. Today I met a person, she/he gave me a book I wanted for a long time. I am 快乐的/高兴的。 In a bus, a girl give her boyfriend a kiss. The boy feel 快乐的/高兴的 and 幸福的. The boyfriend is 幸福的。 Hoping my explaination will help!

And.... what`s the difference between 可以 and 高兴 ? I thought both means happy. Is it correct?