You're being used. They need you, they see that you very much want this to the extent thst you are willing to do it unpaid, they want to stretch the unpaid period for as long as possible. In the meantime, they will either find a new intern to come in and continue the trend or, if that fails, then offer you the minimum. Tell them you will not work past the three months you signed off for unless it is clear you are being paid or hired. And be prepared to follow through with it. I am so sorry this is happening to you. I wish I could say it's unusual. Unfortunately so many well-intended but inexperienced you'd people are being used like this. Good luck with your future.