Which one is the correct sentence?

All of these sentence are correct in their own sense. It is not possible to tell which of these is correct unless you provide the context/source.

Even though I couldn't speak to you, I'll be missing you.

Meaning: Even though you couldn't speak, you will be missing her/him. You didn't had the chance to talk to her/him earlier, and so you couldn't talk to her.

Even though I can't speak to you, I'll be missing you.

Meaning: This means that you can not talk to her. Either someone else restrained you from doing so or for some other reason.

Even though I don't speak to you, I'll be missing you.

Meaning: This means that you don't talk to her. It is your choice to not to talk to her.