So it hast already been three months, I can't believe it. Time flies by.
These three months have been the busiest in my entire life.
And I'm very tired because I have three long schooldays. この表現は自然ですか？

@4nn4 I can understand everything you've written, but look at where I have changed your text, and corrected the grammar:

"‎So it hast (<-- "has") already been three months, I can't believe it. Time flies by.
These three months have been the busiest in my entire life, and (<-- remember to never start a new sentence or paragraph with "and"; it is only used within a sentence) I'm very tired because I have three long schooldays (<--"school days" is correct because the words are separate).