SPEAK, SAY AND TELL COMPARED

Clearly there isn´t a great difference in meaning between these verbs so it´s all a question of use.

When we employ 'tell' we generally say who is told something. There is a personal object, so: 'Can you tell her to be sure to arrive on time?'

When employing 'say' we don't usually say who is told. As an example, you might say 'please say exactly what you mean'. And if we do want to say who is told, we need to use the word 'to', so: 'She said to me that it was a difficult test, and she wasn´t exaggerating'.

There's also another limit on the usage of 'tell'. We only use 'tell' to mean instruct, inform or advise. 'I told them not to be late'... that's an instruction or vague imperative. My grandmother always told me not to marry before having a career.´´ - advising or instructing me.

'Say' can be used for pretty much any kind of talking. But, the following are three examples of when you can´t use 'tell':

He said 'What have you been doing lately?'
So I said that she should never have married him.
David said 'How much do I owe you for your help?'

We use 'tell' without a personal object in a few expressions. These are fixed expressions such as tell the truth, tell a story, tell a joke , tell the time and tell the difference.

And we use 'say' before words such as a word, a name, or a sentence, an example being: 'I promise I won't say a word to anyone.'

That covers the basics of 'tell' and 'say'.  

´Speak´ is often used with the sense of 'talk formally', and when we do use 'speak', we use the word 'to' if there's a personal object. So we would say, for example, 'I really must speak to them about the new timetable' or 'The HR manager spoke to the staff about the problem of punctuality.'.

It is of course also used to describe linguistic ability. ´Do you speak French? Or ´Does she speak German?´

 For a much more detailed analysis of this distinction, see ´Speak, say, tell and talk for the advanced student´