I'm curious, why is Theresa May #3? I've never heard of her.
She doesn't seem as fond of cameras as her peers
The simple answer is she's usually third in the polls amongst Tory voters and the general public, although she has risen to 2nd place in some due to Osborne's support for Pension Reform (nice way of saying cut them to save money)
But to summarize, she's got skills, experience and networks:
77-83, works at the Bank of England as a finconsultant and senior advisor
86-94, becomes Councillor for the Borough of Merton
97, after two failed bids at becoming MP, finally becomes MP of Maidenhead. Once in Parliament, she becomes a member of Hague's frontbenchers (Hague then being the leader of the Tory party).
99, she joins the Shadow Cabinet as Shadow Education and Employment Secretary
01, the Tory party is obliterated by the talented Tony Blair, May survives this murder and remains in the Shadow Cabinet
02, she's appointed Chairman of the Conservative party
03, becomes Shadow Secretary for Transport
04, becomes Shadow Secretary for Culture, Media and Sport
05, Cameron appoints her Shadow Leader of the House of Commons
09, becomes Shadow Secretary for Work and Pensions
10, the Liberal Democrat attempt to decapitate the Tory leadership by unseating May at Maidenhead fails, with Theresa May completely unchallenged
10, after Cameron becomes Prime Minister he appoints Theresa May as Home Secretary - there are three Cabinet posts that are massive responsibilities (even if the EU has eroded the importance of Home Secretary and the Foreign Office has paradoxically lost importance to the UK but gained European importance, but I do digress).
15, May is reappointed to Home Secretary again, retaining her post. May has from 10-16 been the executive in charge of matters of policing, national security, migration and citizenship. Furthermore, if Cameron did not have the help of May and Osborne, he would have already lost the EU referendum campaign by now. I think it's rather interesting to note how quickly she rose up the ranks after 2001, it seems after the chaos of their defeat she was quite the busy worker!
Used to be that it was Osborne who was the favoured Tory candidate (that wasn't Boris) but he got rekt by the IDS of March. Furthermore, he could end up getting fatally wounded in the EU referendum, or Boris could get fatally wounded in the EU referendum - or both, which would only leave Theresa May as a viable candidate (unless some dark horse runs up just as Boris did for London). As it stands I reckon Theresa May is close to consistently topping Osborne in the polls (I just want to see if she's got staying power first). A lot of the dark horses I would've thought stood a chance like Sajid Javid or Gove don't seem to like their odds, and Hunt's been slaughtered over the doctor pay packages so it makes sense that he stood out this one
Boris is #1 despite not holding a senior office post because he usually polls as twice as popular as his rival colleagues amongst Tory and general public voters
*EDIT
Oh yeah, I also forgot, in regards to Labour boosting their membership they had loads of entryists from other political parties signing up, so how successful old labs strategy was is doubly in question