"alright" , "ok" or "fine" all mean " I comprehend/ understand"

That is okay with me. Alright, let's go to the Post Office first, then we'll go to the restaurant. I'll agree to that! That's fine! We can go to that Italian restaurant. I'm fine with that! All the above mean the same thing. I agree with that decision; it's okay, fine and alright with me!

No difference, except that "OK", "Okay" and "alright" are slang and should not be used in formal writing unless they form part of a dialogue, etc. "Fine" is a more formal word.