The third fuelcock is for crossfeeding. The Blenheim manual strongly suggests to leave it, unless it is really really necessary. Set mix to 'normal'. (almost all the way down) The manual also suggests to pull the boost cut-out lever and take off with +9Lb boost. But I think this fries the engine way too fast. Also, trim the nose slightly down for take-off, so the tailwheel comes off the ground more quickly.
The Blenheim is very sensitive in the engine management.
I got the Blenheim V manual from somewhere here:
http://www.ww2aircraft.net/forum/oth...-systems-tech/
This is a diffrent aircraft then the flyable Blenheim IV in the game, but the eninges should be the same.
Also, in the cross-country mission, you should reset the trim of your rudder as the first thing to do, before touching anything else. This seems to help!