The differences among these sentences:"i miss you" "i have missed you" "i missed you". Thank you!
Im not quite sure about how to use them...

Question

Answer

I think actual usage examples will better help you understand the differences.
In these examples, "said" means written or spoken.

~"I miss you."
...Said (by phone) to someone who is not there.
...You miss them right now, as you speak.
...Or, said with a qualifier, to someone who has returned:
...~"When you are not here, I miss you."

~"I have missed you."
...Said to a person who has now returned.
...Said to a person who is still away.
...You are stating your feeling up to that point in time.

~"I missed you."
...Said to a person who is now back.
...Your feeling is in the past.
...Or, said with a qualifier to someone who is still away:
...~"I missed you for a long time, but not anymore."

Ah, the beautiful simplicity of a single " 我想你。"

Answer

I miss you - present tense- u miss now!
i have missed you -present perfect- from few days till now, he/she is still absent! 
i missed you -past simple- u missed him before when he/she wasn't here!!

Não perca a oportunidade de aprender uma língua no conforto da sua casa. Navegue pela nossa seleção de professores de línguas experientes e inscreva-se na sua primeira aula agora mesmo!