I think she's set them up unless one or more of them comes up with a good solution to this mess (I'm guessing she thought Davis was the most likely to do this - she must be very disappointed so far).
And surely her preferred solution has to be that between them they produce something. Strikes me a something of a 'working towards the Fuhrer' situation (as described by Kershaw) - ie May herself does not involve herself in the details of Brexit, doesn't care particularly what the details are so long as the problem is sorted, and sets these three up against each other to come up with those details.
Problem is, if all three fail, she may well have set them up and be able to blame them and get rid of them, but the problem will remain unsolved.