Why my land rover discovery 3 turns over but wont start?

First, rule out the immobilizer on Your Land Rover Discovery 3; if the anti-theft system has been triggered, You might see this very behavior, as it's cutting out after the crank. The next thing is to check the crank position sensor; if this is failing, it'll typically not allow You to start when the engine is hot, but You'll be able to when the block has cooled down. After that, check the supply fuel and supply spark. That is, see if the fuELRelay and pump are actually sending fuel; the engine stalling out immediately could indicate it's starved for something to actually burn. The spark plugs not sparking means that the fuel's getting there, but not getting ignited, which results in the same end result: A vehicle that'll try to start and fail. Finally, if You've verified those systems are fine, You might actually have a fuel pressure related problem, or a clogged intake manifold; this is pretty rare though.