Do you have a supervisor/tutor with whom you could discuss these titles? Essentially it's usually through having a chat about a proposal that supervisors are able to estimate whether a question is small enough/big enough for a PhD. For example, your third question might be feasible as a case study of one or two institutions, but if you wanted to assess the general case that might be beyond the scope of a one person project.
You essentially need to find a question where you can find a suitable method for testing the hypothesis rigorously in a suitable timeframe. That's where more experienced researchers should be able to guide you.
Also be aware that it's better to choose a question to which the answer will still be of interest in 3-5 years time. E-learning seems to be going through a transition stage with MOOCs (though whether they are sustainable is another matter), so factor in what kind of issues people are going to be likely to want to know about in the future.