DÊ RESPOSTAS - English

Tell usually means you are talking to somebody directly usually, but not always an order. For example you tell someone to "do the dishes". Say is less direct and is more open to other people to respond to. For example you say "The weather is nice today". Speak is generally one way. For example "I'll have to speak to that boy, he's getting lazy."

Somehow, for me this was always useful:
You tell a story.
You say something to someone.
You speak a language.

Which is, of course, not even half right, as drewccapp explained it quite well here, but - at least - it gives an idea of the "feeling" for these three words.