Choice between different PhD Positions
Is there a deadline by which you must accept the two offers you currently have? If so, send a polite email to those institutions explaining the situation and asking for an extension on the deadline until you hear back from the third. I expect they will be amenable to your request, given the circumstances.
If you accept one of the current offers without waiting for the outcome of the third, I don't think you will be very happy. You must pursue the position you want the most and feel is the best fit for you. Your PhD will be much more productive and enjoyable that way.