Student or Learner
Are these sentences OK?
I have left three messages for Mary and till now she she has not returned my calls.
It's OK but I see no reason to complicate "I've left three messages for Mary but she hasn't called me back/hasn't returned my calls yet." I find the use of "till now" in the middle of the sentence a little unnatural.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
I see no need for "till now" or "yet" at all. She hasn't returned my calls.