In order to qualify as a main home you have to live in it with some intention of permanence.
There are cases where people have lived in a property for days and qualified for PPR (I think there was a case where someone bought a house and moved in with the expectation of being married, that then collapsed and he moved again and qualified for PPR) but there are also cases where people have "lived" somewhere for long periods but the courts have ruled that it didn't count for PPR because they never had the intention of it being permanent.
The longer you live there before you put it on the market the better your chances of qualifying for main home replacement. Also if you let your current home before starting to market property B then that's another bit of evidence that it really is your main home.
There is no hard and fast rule unfortunately. I belive this is one of the cases where you can ask HMRC for "pre-clearance" to confirm whether you qualify for the main home replacement exemption.
1. Assume you let property A and move into property B.
2. assume you cannot afford to buy property C unless you qualify for the main home replacement exemption.
If HMRC allow main home replacement exemption it then all well and good.
If HMRC don't allow the main home replacement exemption then you cannot move and property B becomes your permanent main home - and therefore you qualify for main home replacement.
I am neither an accountant nor a lawyer.