Ignoring the difficulty of it, I imagine you would need to show sufficient evidence that you could handle it before they let you dive into it. I also imagine, unless you have extremely odd circumstances, this evidence would be hard to supply.
I also, personally (as obviously I have no say on whether or not it happens), don't think they would. As once they let one person, where do you draw the line for everyone else doing it?
May I ask why you think you could do it in 3 years?
Of course, there is no harm in enquiring with the maths department. Only they can give an actual answer.